Could a blown wing ever be powerful enough to lift an aircraft at zero forward velocity?
Technically speaking, yes, it's possible, but wildly impractical.

Of course, as usual, Jan Hudec's explanation of the physics is completely correct. Whatever is blowing enough air over the wings to fly would generate an insane amount of forward thrust, meaning that you wouldn't stay at zero forward velocity for long.

However, it's completely possible to apply another force in the opposite direction equal in magnitude to what would otherwise be the net forward force produced by the engine(s) blowing the wings. This would yield a total net forward force of zero, thus no forward acceleration would happen, allowing you to take off at zero forward velocity. Of course, you'd want to make sure that whatever is applying this force isn't also blowing the wings in the opposite direction, otherwise you lose your lift.

Some possibilities for providing this force:


Normal jet engines mounted backwards
Normal propeller engines with negative pitch
Rocket engines mounted backwards
A (really strong) tether
Wheel brakes (this only works as long as you're on the ground, of course... you'd start accelerating forwards very quickly as the wheels break free from the ground, but you could technically leave the ground with zero forward velocity.)
A really big machine gun, such as the GAU-8 Avenger or Gryazev-Shipunov GSh-6-30
Lots of AK-47s ('lots' being a number roughly equal to the net forward thrust of the engines in pounds divided by 13)


This is roughly what the backwards-mounted rockets would look like:

C-130 Rocket Assisted Landing
Source: YouTube

Another possibility to technically meet your requirement of taking off at zero forward velocity would be to simply start off rolling backwards fast enough that you would accelerate forwards to zero forward velocity at the moment the tires leave the pavement. :)

Of course, as you've hopefully figured out by now, that all of the above are wildly inefficient ways to take off. They would, however, technically work. And, of course, in the case of the GAU-8, whatever is in front of you during the takeoff is going to have a very bad day.