Irrespective of its state, blood is referred to as a connective tissue because it has qualities of connective tissues. For example, blood comprises cells, a non-fluid matrix (plasma), and fibers -three basic properties of connective tissues.
Also, like connective tissues, blood acts as a connection between different body parts and allows for the cross transfer of secretions and chemicals.
Like connective tissues, blood also acts as a packing tissue. i.e. it forms a packing that surrounds the organs in the body.
Last but not least, blood has fibroblast -the most valuable cell that is a part of all connective tissues.