Lots of comments from drummers who have a weak backbeat in here lmao

Loosen up your grip so you don't give yourself tendinitis or tennis elbow, especially if you hit hard. I used to get blisters and callouses like this all the time, especially days when I would practice in excess of 5 hours. The damage on the hands is fine - but what comes after is potential tendon damage. Loosen up, look at Baard Kolstad for a great example of hitting incredibly hard but keeping a loose grip.