My grandma has a bedsore post op that has become black in some areas; what should I be concerned about?

Depends. It is impossible to tell you what the black areas mean without seeing your grandmother and the sore...It could mean infection or it could be a small area of dead tissue that is of no consequence. She should be examined by her doctor or by a physician who is familiar with pressure sores, their management and complications.
Infection. Bedsores, or pressure ulcers result from prolonged pressure on a area of skin. There are different stages of bedsores (1, 2, 3, 4) and are based on how deep the ulcer is. If there is black areas, this is a sign of necrosis and needs to be treated imminently. Depending on how deep it is, infection of the bone may happen (osteomyelitis) which is very serious. Please have grandma examined by a doctor.
Infection. . Especially if wet or draining. Have the home health nurse take a look or your md. If redness is spreading or she is looking sicker call for ambulance to er. .
Wound care. Blsck skin is generally dead and should be removed to minimize the potential for infection. Discuss with a wou d care specialist.