It's not actually true that only direct heals trigger it. Any heal over 1500 does, including Healing Rain.

I healed at ultraxion fight 3 times now, 2 of them were in LFR and once with my guild in 10man. At LFR i took the blue crystal and used HR + CH like you said and it was satisfying enough for me. But with my guild group they've insisted that i should take the Green crystal because it is said to be best for resto shamans...

The truth is i'm not sure how to heal with the green crystal. I think you should spam Greater Healing Wave with it? So that's what i did (with some healing rains) and even though my final HPS was not so bad i still can't see how this is better than the Blue crystal.

If this was the right strategy with Green crystal (spamming GHW) then the reason for "it is best for resto shaman" is because we have the most powerful single target heal in game. So my question is; can anyone confirm that blue is not better than green for shaman?

You heal with green the same way you heal with nothing. Your healing is doubled. It's the same as red, but instead of x2 on anything hit, the extra value is split across the raid. It's not a smart heal, so where you start the target of your heals is where you normally would. Green healer acts to smooth out the overall healing.

The problem with blue crystal, is that just about every healer but a druid wants it, because its "best" for them personally. What matters though, is what is "best" for your raid. In that situation, pally holy radiance, and priest prayer of healing outshines the CD locked shaman and druid.