It consist approximately 7 percent of body weight.
By definition organ is composed of multiple tissues. Blood is a fluid, a circulating tissue.
Therefore can we call this fluid system a liquid organ?
Blood is considered a type of connective tissue (sometimes). However, an organ is formed of multiple different tissues. Thus, blood is a tissue, not an organ.
While organs are generally considered to have a single, specified function (or perhaps a group of closely-related functions), blood (the fluid inside the vessels, not the vessels themselves) has many different functions:
So, I don't think blood should be classified as an organ, just because of its multitude of uses. Instead, it is vital for the proper functioning of pretty much all the organs in the body, and allows for connections among them.