Jamgon Kongtrul, Creation and Completion:

The clear form of the deity is the luminous appearance of your own mind
And the unclear, unsatisfying experience is also your own mind!
So also, mind is the one who desires clarity and tries again,
And mind is the wisdom deity and guru.
Everything is mind's appearance, and yet mind itself is uncontrived.
The beauty of this ultimate essential point of the approach of the two stages
Is that no matter which of the many creation stages you do,
If you apply clear awareness and mindfulness that is merely undistracted,
When the meditation is clear, it arises as clarity-emptiness, and when obscure, as obscurity-emptiness!

Ay! What does that last line mean? I take it to mean that if the visualization "works" -- if you see the deity clearly and vividly -- then you are correctly perceiving its emptiness (because if you are mindful and undistracted, and yet still can create this image, you must be able to perceive its emptiness. We'll leave aside for the moment the ugly question of "how do you know if you're undistracted?") But even if the visualization doesn't "work," you are still seeing your own mind, and what's more, you're seeing its emptiness. You can't possibly mistake the unclear unstable pictures presented by your confusion as innately real things, and you have to be aware (since you're working hard and failing) that they're productions of your own mind. So you're practicing the perception of emptiness in either case.

Very, very important to bear in mind the caveat that for this to be true -- for this lovely heads-I-win-tails-you-lose situation to arise -- you have to be applying clear awareness and mindfulness. Which is why the grounding in Shamatha is essential.

I hope that's what it means, because my visualizations have been so obscure lately that sometimes they're barely there. But I think I'm still pursuing them with decent meditative concentration. So if I understand this right, I still am garnering the benefit of realizing "obscurity-emptiness"!