You aren't quite right. You don't provoke with melee or close attacks. Only ranged and area attacks provoke. Burst or blast doesn't matter.You don't draw an Opportunity attack with a blast or burst.
Ranged powers do.
Does the blast work even if the ally is invisible?
One of the (many) subtleties to note when choosing / analyzing powers is how their range works and how that interacts with the Opportunity Attack rules. I can't count the number of times I've looked at a power and done this: "Oh! Nice, I'll be taking ... to hell with that, I'm a melee guy and that power will get me stabbed."
Concealment, Total Concealment, Line of sight, and the Hidden status do not matter towards Area or Close attacks.
Can anybody explain to me what the point is of Healing Word being a close blast power ..
I've been wondering why these powers are not Melee 5 Instead of Close Burst 5, one creature in burst. This might be the reason why. A more likely reason is simply that Melee 5 is a very ugly way to describe something (even if it works perfectly well in the game).