As a translation, the word simply means "pretty boy", but that doesn't tell you a lot, does it?
What it really refers to, is a special type of man you'll meet in manga (and not only those mainly for women) and even in old legends and stories from Japan. The "bishounen" is a man who usually looks slightly (or more) feminine, but isn't necessary behaving like a woman. "A man as pretty as a woman" is a description even given in old tales. It refers to that slightly effeminate type of man, but doesn't mean the person is nice, helpless or friendly. A bishounen can be everything, even nasty, dangerous and outright evil.
In fact, I found the link to the "Black Widower" stories about two youthful vampires on a website dedicated to bishounen - which also featured Angelo, one of the main characters of the "Black Widower" series. Angelo (and his lover Weasel) are anything but nice and helpless - they're predators.
Bishounen are a Japanese speciality ... and it helps to know about them before you watch a Japanese movie (as I did with "Werewolf Warrior"), because in Europe or America we tend to think a "feminine" man has to be weak and a good-looking man has to be good. That's not always the case.