There are more principled ways to do that than a blacklist, though, like looking at current frequency compared to typical frequency. That way even something commonly mentioned could still genuinely be trending if it gets mentioned much more than usual on a particular day.
(It's possible there are scale-related reasons that make this infeasible, though.)
Just not tracking the frequency of the most common words that will never be in the trending topics is probably a big performance win. I'm sure there is an additional blacklist that includes stuff like "and" and "the".
