You see, the thing about culture is that it's rapidly evolving and not necessarily rational (which is perfectly fine). If the word "mainline" would become associated with nazis commonly enough, I'd definitely want to avoid its usage despite of there being nothing wrong it, just like I'm not going to use swastikas just because it's an ancient symbol of well-being. So no, "blacklist" isn't racist, but if we can reduce the ugly associations that our language may be implying, then insisting on not doing that surely doesn't present you in a good light.