My mother is (90)years old her tongue has black on it she have surgery five to six weeks ago someone told me this can come, from (i.v) or antibiotic .?

Maybe thrush: People exposed to antibiotics or those with depressed immune systems(say from cancer) may develop an overgrowth of yeast in the mouth; this is called thrush. It can be treated with oral lozenges or mouth swishes and sometimes may need an oral anitfungal agent. All require a prescription from your health care provider.

Black tongue: I suspect your mother has black hairy tongue. Black, hairy tongue is a temporary, harmless (benign), painless oral condition that gives your tongue a dark, furry appearance. The distinct look results from an overgrowth of bacteria in the mouth that can result from being on antibiotics for an extended period. Encourage her to brush her tongue and rinse frequently.