First of all what kind of cells do you stain, and which magnification is used in this photo? Normally, trypan blue is used as a viability stain in combination with a hemacytometer or an automated cell counter. After staining, dead cells are stained blue because trypan blue can only permeate damaged cell membranes. Viable cells remain unstained. The count must be performed after 3-5 minutes after staining since the cell start to die quickly when prepared on the the slide.