My Seroma on Lower Back is Getting Larger, Hardening and the Area Has Started Bruising? (photo)

Seromas do not cause bruising.  You might have some residual blood in the tissues that is breaking down and spreading throughout the lympahtic system of the skin.  Hardening in the areas is normal as the fluid has swelled the local tissue, and should resolve over the next several weeks.  A hematoma could be present if you can actually feel a discreet mass.  I would have your back evaluated by your surgeon, or seek a second opinion from a board-certified plastic surgeon.