I think the title sums up my predicament well, but if not, then here:

I tried adding the term "luka_(artist)" to my blacklist, but when I revisit the posts page, I still see art by Luka. How can I get this working for me?

Works for me. Are you sure you put it on its own line, with no extra characters either before or after it?

Try it now.

butt penis luka_(artist)

is different from

butt
penis
luka_(artist)

the first one only blacklists images which contain all three of those tags.

the second one blacklists all images with any of those tags.