Kineticist using Water Dancer Blasts to qualify for composite blasts

[ErichAD]

Is it possible to use the kinetic blast to fulfill the blast requirements for combined blasts or for other abilities that ask for either the elemental focus or an elemental blast as prerequisites?

The water dancer archetype specifically grants the "elemental focus ability of the kineticist class" and "kinetic blast feature of the kineticist class", so this should work barring some specific prohibition.

I realize that you don't have access to combined blast till you get the expanded element feature, and I also realize that the identical abilities don't stack unless they specifically call out that they stack so we aren't combining levels for advancing the blasts. I'm only wondering about the prerequisite thing.

[Shiroi]

I think if you multiclass kineticist they would stack up to full level for advancing the blast and level -2 for the talents if I read that right.

It should qualify for composite blasts yes, though you are right about neededing expanded element to actually use that.

Usually when someone gets multiple sources of something (see bloodlines) they are forced to 'agree' with each other, so I feel like while RAW you could get the hydro blast here at first level and the electro blast at second level and air blast at 8th for 3 blasts and the ability to composite them in any combo, I'd ask as a DM that your first choice of blast in Kineticist will 'agree' with this archetype and choose hydro to prevent cheese.

This would be a DM discretion moment to overcome an odd rules issue, and also would be possibly handwaved if your build didn't otherwise look overpowered compared to others at the table, so if that's what you're trying to do I'd just discuss it with your DM. I see no reason besides table balance that it doesn't work mechanically, your mileage may vary and someone else may see other rules.