I don't know about other countries, but in Germany, Will Smith is insanely popular. His movies usually jump straight to the top spot the way they do it in the US. In terms of marketshare, his movies probably perform the same way here.
I have no doubt, however, that movies that are aiming at Afro-American viewers don't do as well. Not because of racism, but because they're not really culturally resonating. We've had a thread here debating about how Tyler Perry movies don't really seem to appeal to 'white people', whereas 'black people' tend to find them hilarious.
While we have people from all over the world living in Europe, but it's all very fragmented. And those who are of African heritage don't necessarily know or like the people who are really popular in the African-American community.
I'm not saying that racism and psychology might not be a factor here and there, but I think saying "less accepting of black stars" is kind of framing it the wrong way because some of them simply are not stars over here. Might as well say that the US audience is less accepting of European movie stars.