Looking for a movie my mom watched in Mexico theaters 1975-1978, Horror Black and White

Sorry, it sounds like something Vincent Price would have acted in. But you never specified:

1. If the movie was contemporary of '75 or showed old films
2. If the movie was strictly Mexican or dubbed American

If it's a Mexican movie, it's most likely it was never shown in USA, thus very few people would be likely to have seen it here.

Try going down Vincent Price's filmography, see if anything rings a bell. Otherwise, ask your mom which Mexican actors were popular in her era in doing scary movies and go down their filmography pages. I'm sorry no one else replied but it's probably by respect, so as not to clutter your page saying "IDK". I've noticed many people aren't respectful enough to come back here and thank the person(s) who found their title, and worse: they never mark it as solved. So if people see lots of replies, they may not click, assuming it's solved and you weren't respectful to users here to mark it as such. 

Anyway, hope this provides *some* help and this 3 months old post is now bumped to the top, too, so maybe someone new will know it. Good luck! :)