Blood is considered a tissue because blood-

Blood is considered a tissue because it is composed by a different amount of cells.

Explanation:

If we remember the basic definition of tissue we get an answer related to the association of cells to create a single object that carries out a function to permit a body's process. So, the first thing blood has in common with that tissue definition is that it is composed of cells. The second one is that blood is necessary to carry on a specific body process, and finally because even though it is a fluid it needs to be together to work properly if we segment it will start failing.