Why do my fingers blister instead of callus?

When I started playing, I got blisters on all of my fingers and those blisters turned into calluses eventually. You can try putting band-aids or something on your fingertips, in the meantime... if you still want to keep playing. This might sound weird, but I put clear fingernail polish on my fingertips once, when they were blistered, so I could keep playing. It actually helped!