First of all, "Celtic blood" and "Celtic DNA" are not something that exist. Quite frankly, that is a white supremacist idea (unfortunately a lot of those seem to get into Celtic-related spaces...)
At it's most sinister, blood percentage is used in places like America to rob Native peoples of their Native identifies if they have below a certain percentage of Native ancestry. Regardless of if they've lived their entire lives brought up by other Native Americans and are very much a part of their culture. The ultimate aim of this is to completely erase Native American cultures, languages, histories, and anyone who identifies with them. Which is genocide.
I won't tolerate those kinds of people who love to talk about their "Celtic warrior blood" or whatever when that ideology lines up with fascism and eugenics.
Your lived experiences with a culture are what make you a part of said culture, not what's in your DNA. Modern Celtic identity is based on the presence of a modern Celtic language, not on DNA.
It is very frustrating when I see Celtic diasporas (mostly Irish/Scottish diasporas in America) claim they're allowed to call themselves Irish because they have "10.5% Irish blood" or whatever, but then turn around and say that immigrants who actually live in Ireland are not really Irish, or that the children of immigrants who have lived in Ireland their whole lives aren't really Irish either.
I identify as Welsh because I was born and raised in Wales. Quite frankly, it would be weird if I didn't identify with the country I've lived in my whole life. But that doesn't mean I can't also identify with my family's cultures. My family are Cornish, Scottish, and Irish, and I identify as Cornish/Scottish/Irish diaspora because I was raised by my family from those places. I do not identify with those places because of my "blood percentage".
My mam is from Scotland and has an Irish mother and a Scottish father. She also identifies as Welsh because she lives in Wales and it's her home. She has a right to learn Welsh and to call herself Welsh. I also have family in Wales who weren't from Wales originally, and who still don't identify as Welsh. And that is entirely their own choice.
I also have an English great-great-grandfather and an Ulster Scots great-great-grandfather. Whatever "percentage English" or "percentage Ulster Scots" that makes me, I don't care. My English and Ulster Scots ancestors passed away long before I was born. I wasn't raised by them and I don't identify with those cultures. I identify as having English and Ulster Scots heritage, because they are undeniably part of my family history, although they are not really that relevant to me. My English great-great-grandfather moved to Ireland after the famine, and my Ulster Scots great-great-grandfather moved to Scotland around a similar time. Obviously this was long before I was born, and I didn't know them at all. I haven't had any relatives in Northern Ireland since pre-partition, and the culture of the north has changed a lot since then, and I'm not going to claim I somehow have innate knowledge or am some sort of authority on modern things like the Troubles.
The Celtic Nations and languages are for everyone, whether they were born here or if they chose to make a Celtic Nation their home later in life.
We can't cry about how we are oppressed, and then turn around and act absolutely vile towards other minorities.
We can't cry about how hardships in our Celtic Nations forced people to emigrate to other countries, and then turn around and get angry at immigrants coming to the Celtic Nations who are also looking to escape hardships in their home countries.
How hypocritical is that?
My mam's side of the family have only been in Wales since the mid-1980's, and my dad moved later, but because I am white I am seen to "belong" to Wales more than non-white people. I know non-white people who are first language Welsh speakers and whose families have been in Wales for much longer than mine. But their Welshness is brought into question a lot more than mine is. Both them and me are Welsh. Someone who moves to Wales tomorrow and makes this country their home is also Welsh and belongs here just as much as the rest of us.
Although I have had the odd person be weird to me about my cultural background, it's not anything like what I've seen non-white Welsh people receive. It puzzles me how other white people in Celtic Nations can claim they experience racism, when surely they can clearly see how much worse non-white people in Celtic Nations get treated. Do they forget the word xenophobia exists? Or even anti-Irish sentiment or Celtophobia? At worse, white Celtic people claiming they experience racism are actively making it harder for non-white Celtic people to talk about their experiences of racism within the Celtic Nations (that they receive from white Celtic people).
How are you not aware of what other people in your own country are experiencing? Are you really such a self-centred hypocrite that you'll (rightfully) complain about how people ignore the oppression that Celtic Nations and Celtic languages have faced, but then ignore minorities within our nations who are also suffering?
And what does "(whatever)% blood" actually mean practically for you? Culture isn't passed down through DNA, it's something you usually learn from the people raising you (and the country you live in, if the county's culture is different to your family's). A couple of times I've had people tell me I'm not really Welsh even though I've lived my entire life here, just because I was the first person in the family born in Wales. The blood percentage model leaves no room for my Welshness and my lived experience being raised in Wales, just because I'm not "ethnically Welsh".
When I get called "half-Cornish" because my dad is from Cornwall, what does that even mean? Which half of me? People with multiple cultural identities like me should be celebrating them all, not splitting ourselves in to fractions and percentages. We should be celebrating our abundance of cultural experiences and connections.
If you are a member of any Celtic diaspora and want to identify with that place, then go ahead, but you need to actually put in the work to be part of that culture. Learn the history and the language, read the literature, and very importantly learn about the modern culture of that place especially if you have no living relatives from there. The culture will have changed a lot if your ancestors emigrated 100 years or 200 years or however long ago.
Don't just say you're Irish-American/Scottish-American/etc as some sort of claim of being a minority, while putting in absolutely no effort to be a part of or to help save that oppressed culture that you claim to care about. Being a part of a culture means that you have to do the difficult things that are also part of it, not just the easy things that benefit you or that you can use to seem more "interesting" or "exotic" or "minoritised" or whatever.