I think I have read somewhere that maximum 20 blogroll links can be added to a page. Google also recommends to not have more than 20 links. But I have seen many blogs who have way more than 20 links, I don't understand how they still survive with Google?
Black hat perhaps? I myself am interested to know more about them seriously but haven't interviewed anyone. Google has advised not to have more than 20 because they already know that that would give us bad effect for the rankings and all. Just stick to 15 - 20 just to be safe.
It purely depends upon the blogger discretion. I could see websites giving away over 20 links in their blogroll