Blindness is the only effect of the power that sustain can key off of. If sustain does not extend the blindness, then sustain has no effect active that allows you to activate it. Sustain is a definition of duration, not merely a line in powers that lets you do cool things.
In this case, it is exactly that.

What you are sustaining is simply the ability to
keep sustaining it, and, thus, gain free attacks.
You have hit the problem with the power design - it doesn't actually list an effect to be sustained, which is typically what sustaining is for.
The intent of the power is pretty clear, reading the flavor text:
"
Flaring points of star-bright light encircle your head. With each gesture, a painful point of blue-white light leaps away in a halo of choking smoke toward one foe."
Clearly what they are aiming for is the idea that, as long as you sustain the power, you retain your 'crown of stars' - and the act of sustaining it is what fires off the lights at your enemies.
As written, it... mostly works. Which is to say, there are 3 or 4 different ways to read it, none of them work perfectly well with the rules, and this interpretation is the one that both fits most smoothly and matches the description of the power.
The alternative interpretations:
1) The sustain effect prolongs the blindness. Except "A power’s description indicates what happens when you sustain it or let it lapse." Extending the blindness is never mentioned in the sustain effect, however. You would be adding a completely new ability to the power, one that is neither referenced within it nor implied by the description of the power.
2) The rules for sustaining refer to sustaining "an effect". The power produces no effect that is referenced in the sustain line, which means you can
never take the sustain action. It might match a strict reading of the power, but also is pretty clearly against both the intent and the description of the power.
3) Or, we have the interpretation that cust-serv offered to the DM, which basically involves replacing what the power actually says with, "
Effect: You acquire a Crown of Stars until the end of your next turn. As a standard action, Make a Charisma vs. Will attack against any target in range. On a hit, deal radiant damage equal to your Charisma modifier.
Sustain Minor: The Crown of Stars persists until the end of your next turn." Which involves even more significant rewriting of the power and, from a power balance perspective, is pretty clearly a useless ability.
Which leaves us with the interpretation I've been arguing for: That all you are sustaining is the ability to, in effect, keep sustaining the power. That if they properly worded it, it would probably read something like, "
Effect: You acquire a Crown of Stars until the end of your next turn.
Sustain Minor: The Crown of Stars persists until the end of your next turn, and Make a Charisma vs. Will attack against any target in range. On a hit, deal radiant damage equal to your Charisma modifier."
That doesn't require any massive adjustments to the power itself, and seems to match the description and intent as well. And is also relatively easy to see why they omitted the additional text, not figuring the need to include an effect that has no actual benefit in and of itself, outside of the Sustain Minor attack.