Would it be racist to assume they were negroes?
No; no, it would not. That assumption is a safe bet based upon pretty widely available facts. Avoiding groups of blacks and the places groups of blacks tend to gather is statistically a wise thing to do. Unless, of course, one is looking for trouble. In that case, then, yes, seek them out and one will not be disappointed. I’ve been around for a while, and know this to be true from direct personal experience.
Our society/government (specifically, democrats) produced what is now four generations of blacks raised without fathers. This is the result.