Like many advertisers, Fidelity Investments wants to avoid advertising online near controversial content. The Boston-based financial-services company has a lengthy blacklist of words it considers off-limits.

If one of those words is in an article’s headline, Fidelity won’t place an ad there. Its list earlier this year, reviewed by The Wall Street Journal, contained more than 400 words, including “bomb,” “immigration” and “racism.” Also off-limits: “Trump.”

Some...