There's a blister on my foot that feels like a callous. There is dark red under the skin. I've had it a while. Is it a blood blister? How do I get rid

Blood blister. It could be a blood blister - these can take a few weeks to reabsorb. Other dark red skin lesions include hemangiomas, which are basically abnormal collections of blood vessels.
Have it evaluated. Some causes of corns or calluses include: friction, pressure, repetitive trauma, and a limitation of joint motion. Some may develop bleeding under the callus resulting in a darkened discoloration of the underlying skin. You may benefit with the use of a special topical moisturize and the use of a pumice stone or an emory board. Consult with your podiatrist and determine the best approach for you.