When to use a blob fly. Blob fly = zooplankton. Zooplankton is a cluster of small microorganisms. They are usually translucent with colorful organs. When the sun hits them a certain way they reflect bright colors. They are abundant in still water like ponds & lakes. They also can be found in hatcheries. So stocked trout could have grown up eating them from time to time. Thats why a trout’s instinct kicks in and they eat a blob. It imitates food they grew up on. I fish them when the water is deep