Delamination is the layering of the polymer as it fills and packs the mold. With this defect, different unconnected layers of plastic develop and form within the part. 

Blisters are gas bubbles which appear as small bumps on the surface of the part. This is caused by either moisture, air, gases or volatiles being present in the resin or on the mold surface.

Both defects can be caused by moisture in the polymer or material degradation. Blisters are more commonly causes by poor material handling while delamination is more commonly the result of degraded material or high injection pressures.

-Andy