I understand that blocks of transponder codes are assigned to different area controls. I am curious if the block 7701 to 7777 is in use anywhere? Maybe the emergency code 7700 needs some extra protection?
Yes these codes are used. Only 7700 is used to denote emergencies, and usually only if you aren't in contact with ATC, or can't get their attention(frequency is packed). Codes in this range are allocated according to the National Beacon Assignment Plan in the US. There are other codes that are assigned for various issues, 7777 as mentioned in the comments is assigned, and I've assigned others in that range before, same with the 7600 and 7500 ranges.
If you're already in contact with ATC, and they have you tagged, it is very easy to mark you as an emergency and have it light up on everyone in the facility's scope.
Airman's Information Manual AIM 4-1-20 e.2 says, "Under no circumstances should a pilot of a civil aircraft operate the transponder on Code 7777. This code is reserved for military interceptor operations." (AIM 2016 edition, seems to be the latest edition as of 2017 Sep)
AOPA flight training magazine Nov 2017 advises temporarily putting your transponder to standby while changing numbers so that you don't accidentally trigger alerts in towers by passing through the special 7xxx numbers.