On Twitter, ‘blocking’ means you the blocked person or ‘blockee’ cannot see the blocker’s tweets anymore; at least when they are locked in. (If the blocker is ‘shadowbanned,’ then it is also not possible to search for their tweets or their account; even when the person searching for them has logged out).
One arguably asymmetrical effect of Twitter policy is that someone can remain in your follow list, even if they have blocked you.
I found an examples of this very recently.
If you want to stay in my follower list, then don’t block me!
Any followback attempt was met with: “You have been blocked from following this user at their request.”
The responsible thing to do is to block this person, as it is not appropriate to have someone block someone else, and yet have them still able to read all of your tweets. On the other hand, if you really don’t mind that… well, whatever! You must be a pretty chill guy…
Another case, which is arguably more of a gray area, is being followed by private accounts. These are accounts where the tweets are not public.
In such cases, you are free to send a follow request; hopefully, it should say “pending.”
If you send a follow-back request and it is ignored, you may examine each case on its own merits , in order to decide what to do next.