I only almost overdosed while I was in a mental institution and the nurse had dissolved my medication to force me to ingest it (I guess I'd refuse pills back then)and since it tasted so awfully bad eventually I threw up. He then injected me but the meds(that include risperidone) had already been absorbed by my body, at least partially so I started trembling and feeling really ill until another nurse came up to check on me and brought some sort of antidote to inject me. I guess I was about to lose consciousness.

I'm not sure if I had black outs because of risperidone, I mean taking my pills while outside that mental institution, but yep, I find my memory really faulty(I lost the sense of a timeline regarding manic episodes) and there are many things I don't remember when I was using a high dose of risperidone, but then again, this dose was for my mania and I'm told that mania makes us unable to really "catch" what's happening within our surroundings and ourselves and that plenty of stuff just isn't recorded in our brains. I can't tell if it's your case but I'd question your doctor about it.
It's really a terrible feeling to be left wondering about things we really can't remember and people say we did it. :/