As you noted, blister fluid is clear, not red. So, rupturing of blood vessels is an unlikely explanation for the fluid accumulating in the blister because then it would be filled with blood. (On the other hand, "Blood Blisters" are red and filled with blood). You've already hinted at what the actual process might be. In your question you noted that "serum from the blood" collects in a blister. The serum is the clear liquid secreted from blood vessels that have been damaged. The kind of biological mechanism that would allow the water and molecules that are present in serum to pass across the blood vessel walls while leaving the rest of blood components (such as the red blood cells) behind is called diffusion. Diffusion across a cell membrane is an important mechanism that allows water and some molecules to move across the surface of cells.