Shadow Blend from Lords of Madness pg 168 says:
In any conditions other than full daylight, a shadow creature can disappear into the shadows, giving it total concealment
The SRD states that
Total cover or total concealment usually (but not always; see Special, below) obviates the need for a Hide check, since nothing can see you anyway.
Since when you are hidden from an enemy he is flat-footed against your attacks
Rules Compendium pg 92
If you're successfully hidden with respect to another creature, that creature is flat-footed with respect to you. That creature treats you as if you were invisible.
Does shadow blending make my opponent flat-footed?