I just used the black resin on a couple inter locking parts that will later be molded plastic and the parts are very brittle breaking real easy. I cured the parts using the cure time and temp off the web site.
Standard resins are indeed very brittle, with Black and White being the most brittle, Clear the least and Grey somewhat in the middle.
For functional parts that clip together you either have to make adjustments to the models to avoid too much reformation or print with another resin, in which case Tough and Durable are recommended.
normally I use the tough resin but I wanted the parts to look like they would as production part. I use a lot of the tough and clear resins.
I have given up on using Black as results were way too inconsistent and in any case the parts are too brittle for even a prototype, unless I’m the only one handling them.
Tough and a good layer of mate paint from a reputable brand are my go-to. for lookalike functional prototypes.