Hello,
I had that same problem too (not sure whether I tried to add someone before or not) and unfortunately it is not possible to see who rejected your friend request (Facebook does not notify if a request has been rejected).
Regarding the blocking question, the only way you would sort of know is if you are positive a person has facebook, bit you cannot find them. In that case they have either blocked you, or their privacy settings are set to very high so that no one can search for them in a search.
Hope that helps!