Why is blood considered a connective tissue, despite its fluid nature?

Blood is considered a connective tissue because it originates from mesoderm, connects various parts of the body by transporting substances, and, most importantly, it possesses an extracellular matrix. Unlike other connective tissues where the matrix is solid or semi-solid, blood's matrix is fluid, called plasma. 

This plasma contains dissolved proteins, salts, and other substances, in which the cellular components (red blood cells, white blood cells, and platelets) are suspended. This fluid matrix, along with its cells, fulfills the essential criteria of a connective tissue.