The Blob itself is frightening. It is a carnivorous, amoeba-like single celled monster from outer space that devours its victims whole, enveloping and digesting them completely. The Blob's victims are eaten alive, and express genuine terror before they are consumed.
Because the blob is amorphous, it is incapable of expressing any form of emotion. This adds an especially merciless quality to the way the blob murders its victims, which younger and more sensitive viewers may find disturbing.
Throughout the film, a growing blob eats and dissolves citizens. Most of the carnage is off camera, but it may be disturbing to young viewers.