There are many websites online which let one download videos from
YouTube. These breach the Terms of Service of YouTube, which is owned
by Google.
Could Google legally block these websites from appearing in Google's
search results?
Yes.
Google can set up its search results any way it wants. It has no affirmative obligation to be fair, or impartial in its search results, so long as it doesn't commit anti-trust violations in doing so.
It may be unwise as a business decision to do this, but it isn't illegal for it to use its search results as a form of self-help to enforce its legal rights, or the rights of its affiliates, under Term of Service Agreements.
Could it potentially violate antitrust law if they don't block
websites which let one download from other video platforms?
This is unlikely.
The fact that you employ self-help in your own business practices to deal with people who violate contracts with you does not impose upon you a duty to police the entire Internet in cases of contract violators who have nothing to do with you.