What Causes Black, Fibrous-looking Stools After Multiple Antibiotics And Surgeries?

My husband has been on multiple antibiotics, preceding & following Laser Prostate Surgery on March 7 /22 Along with two Hospital re-admissions for infection of Bladder & then Scotum/Testes ... he developed black stools ... which look very fibrous - resembling Cooked Spinach in appearance ) (neg for Occult Blood x 2 -- & does not have smell of Melena ) Any suggestions why this appearance ... & what tests can be done to diagnose same (aside from FOB ) ?? Thanks ... A retired Nurse