Here's why:

* Tissue Definition: A tissue is a group of similar cells that work together to perform a specific function.

* Blood Components: Blood consists of different types of cells (red blood cells, white blood cells, platelets) suspended in a fluid called plasma. These cells all work together to perform vital functions like oxygen transport, immune defense, and clotting.

* Organ Definition: An organ is a structure made up of different tissues that work together for a common purpose. While blood is essential, it doesn't have the complex structure of an organ.

So, blood is a connective tissue, specifically a fluid connective tissue.