M57, what is difference between blister and ulcer in context to diabetic foot problems. Could a blister become big problem.

Yes. In a diabetic something as simple as a blister can be a huge problem if not treated the right way.
Can go together. Blister is a fluid filled sac of skin usually resulting from abrasion/friction. An ulcer is a break down in the surface of the skin and the dermal layers under skin surface; it usually exposes fatty tissue, veins and arteries, muscle, ligaments, and bone. A blister can lead to an ulcer if this treated immediately or properly.
Blister and ulcer. Both can be serious with anyone but particularly diabetics. A blister indicates excess rubbing/contact causing a fluid filled sac. This can be a precursor to an ulcer or infection if allowed to continue. An ulcer is a break in the skin and is graded on the tissue it extends to or through. It is much more serious than a blister and requires medical attention. Good luck !