Datasheet term for transistor power consumption when OFF?
No, there is no such parameter.  At most there will be off state current, ususally called "leakage current" or "cutoff current".  The power has to do with applied voltage, which only you can know.

However, the leakage currents of normal transistors are so low as to be irrlevant in virtually all cases.  If the issue is switching 1 A LED lights on and off, then the off current will definitely be irrelevant.

If you are worried about power consumption, then darlingtons are probably not the way to go.  These have significantly higher on-state voltage drop than a bare bipolar, and a FET can have even lower than that.

It would be a good idea to explain more your overall problem and ask how to solve that instead of asking details about a possibly inappropriate solution.