This is a good point. My gut inclination was to defend him on this point...Afro-Europeans (at least on the mainland) aren't really a thing so most people don't have the context of dealing with racial sensitivity.
But Hrabik has been here for quite a while and should have picked up on it.
I noted it as a possible defence but I don't believe it actually is one. Because regardless of whether or not you are exposed to a multicultural society growing up, part of any person's social education should be the basic concept of decency. The baseline expectation of any well-adjusted person should be not to insult someone for an immutable physical or social difference such as race, ethnicity, gender, sexuality, religion (in the context of being born into a family or grouping that is largely identified as belonging to a particular faith regardless of how devout any individual member is or isn't), etc.
There may not be many black people in the Czech Republic, but it shouldn't take interacting with people of other races on the regular to know that demeaning another human being as if they're less than human for
any reason is repulsive and inexcusable behavior. And it's not like the Czech Republic is a monochromatic country anyway. They have a decent sized East Asian population (Mongolian, Vietnamese, Chinese) and would have some manner of ethnic and religious distinction. Hell, their country is itself founded in part on the dissolution of Czechoslovakia on (at least partially) the grounds of the ethnic differences between Czechs and Slovaks.
In the end ignorance is not an excuse for a lack of decency. But in this case it's even less of one because of Hrabik's integration into North American society.