I had a mole removed for biopsy a little over a month ago, which tested as benign. But it came back just as big and dark. Should I be concerned?

Recurr/nevi. YES! Needs 'complete' excision. Return to your surgeon for repeat local surgery.
Mole. As long as the initial biopsy diagnosis was benign you should be fine. The lesion wasn't entirely removed during the shave biopsy. Ask your doctor what he or she thinks.
Yes. If the mole was not completely removed , it may regrow, especially if it was of the category called "atypical" or "dysplastic"