Is black and tarry stool and abdominal distention following a epidural steroid injection signs of gastric bleeding?

Yes. See a doctor: Dark, black tarry stool is called melena. It is a sign of GI bleeding early in the GI tract like the stomach or duodenum. Steroids can cause gastritis but there are other causes as well. Either way GI bleeding enough to cause melena needs to be investigated. Often a small or medium bleed is just an early warning of a large impending bleed. See a doctor asap.

Yes : Black and tarry stools with abdominal distention is a reason to seek immediate medical attention. It may indicate bleeding and bowel obstruction requiring surgical intervention.