This technique consists of taking a drop of blood and looking at it under a microscope. The intention is to observe the size, shape, and relative abundance or scarcity of various blood cells, as well as to identify any abnormal types of cells that may be present.

This technique is very specific in terms of being able to find abnormalities. Various conditions in the body have different effects on the appearance of the blood at a microscopic level. So it can tell us that blood cells are or not being formed correctly, it can be used to identify the presence of certain pathogens and parasites, and it can let us know, to a certain extent, how blood cells are behaving. It is useful for identifying patterns of function and dysfunction, in the same way that looking at someone's physical appearance or feeling their pulse can be. 

Blood cell analysis gives us an idea of what is there. It can't reliably be used to rule out most things, because the fact that you don't see something doesn't mean it isn't there.