These are SubStation Alpha scripts which can be "attached" to video players to give them subtitles. They were originally from Annapuma's Script Crypt, and were created by Pakman Productions, and translated by "SS". (Note that all of those links are now dead/archived.)
TOW has a list of software that can use these files - I use VLC.
There aren't any "official" scripts from the BH anime published anywhere, that I'm aware of.