Well, that's not really what you want but the outcome is the same.
Can't you just take all blp, convert them to png, and then use something like batch taks in photoshop to do what you want with the png files.
I'm not sure if other free programs like gimp support batch processing, as I'm not an artist and just happen to know photoshop from a collegue at work.
After you're done you can convert them back to blp and use them ingame.
I'm pretty sure there was a programm that was able to convert complete folders via batch script between blp and png.
