Can too much castor oil cause anal mucus discharge? the first 36 hours it was a yellowish color with a little diarrhea, after which, it turned black.

Yes...: Castor oil has stimulant laxative properties causing forceful contractions of the intestine and violent diarrhea. The intestine contains cells that secrete mucous providing a lubricant function. I suspect the castor oil has led to a overproduction of mucous which you are seeing in the anal discharge. If your stool is black - seek medical attention to rule out bleeding from the GI tract.