well it depends on how you understood what i have said..
I think your original post is pretty clear. It's not simply that your Polish boyfriend cheated on you with another girl but that it was with a black girl in particular. I just don't believe you would have given any emphasis to the other girl's race if she hadn't been black.
You mention the word "black" four times including in the title of your post. That is the real issue for you. You know this. It is understandable that it would be uncomfortable for you to admit it since you added qualifying statements in your posting. Of course no one wants to be regarded as a racist. I'm sure you and many others genuinely believe that interracial couples can't have misgivings or indeed prejudices towards different racial groups outside of their relationship.
Although there is a lot of diversity in the Americas, Europe and Australia the same cannot be said of Africa, Asia and the Middle East where most countries are predominately homogenous. Although Western societies have a longer history of having larger diverse populations all of them still display reservations about how far they will interact or enter into a relationship with other people from a different race. Actions always speak louder than words.
The issue isn't about whether or not Polish guys in general have a thing for black girls. That fact that you have a Polish guy now that you say is your boyfriend proves they are more interested in other aspects of a relationship beyond the color of skin.
Your boyfriend just seems to have found something more interesting in another girl which he doesn't see in you. Rather than it being a matter of race it sounds like it has more to do with your individual personalities and commonalities which are lacking. There is a lot you haven't brought up. Maybe you are a homebody and he likes to be more social or there are other controlling aspects of the relationship beyond sex which is causing unspoken tensions and a need to find an outlet.
If you can't talk with him about it then you should just end the relationship in as friendly manner as possible.