Restylane - Dark Area Under my Eye - Tyndall Effect?
It's hard to say 100% from that photo but a Tyndall Effect is typically described as a blue color emanating from the location of the hyaluronic acid filler that was injected. This is a phenomenon of physics and has to do with the manner in which certain wavelengths of light are scattered by particles (that's about the limit of what I know on this, btw). So if the area is truly bluish, then it may represent that.
More likely, though, is that this is...