I agree with you that racism is everywhere, but Boston has some issues that are fairly unique to Boston. First, it is an incredibly segregated city, much more so than most other large metropolitan areas. Second, professional people of color in Boston are almost non-existent, there are students and then there are people of color generally living in poverty in certain pockets of the city (intertwined with the segregation issue), but there simply are not a lot of doctors, lawyers, accountants, etc.. And it is a problem that builds on itself because professional people of color in Boston don't stay in Boston because there is a massive lack of support networks for them. Third, because the "Boston is a racist city" thing gets thrown around more than you hear about other places like Philly or New York or wherever, white Bostonians are tremendously thin skinned about it, and you can see it in this thread (edit: I don't mean to say your post is defensive, it isn't, just using it as a jumping off point). I could see how all of these things could contribute to wealthy, black professional athletes not exactly feeling comfortable in Boston, as compared to NY, LA, Atlanta, Philadelphia, etc. That may not be the same as being harassed by police or hearing someone use the N-word, but you could call it racism and I wouldn't object to the use of that term.
I've posted this before, but this Daily Show clip humorously hits on a number of these points:
View: https://www.youtube.com/watch?v=QtUgq2Q1ivA
Kyrie Irving is a dink and I agree with whoever said that it can be simultaneously true that he's shamelessly bringing this up now for completely self serving reasons AND it could also be true. But pushing back on the "Boston is a racist city" or trying to prove that if it is racist it isn't any more racist than anywhere else is just a bad look IMO. I accept that the city I love has a lot of work to do in this area.