Actually, I think your code is correct (however, i don't understand the "add 1 to index" action)
You just have to change the flash values: write "random(0.1,0.3)" instead of "0.2" in the flash parameters.
The sprites will flash at random speeds.
I think I kind of confused you too with my question
What I meant was that I'm trying to make them flash one at a time, for example:
block(1) -> flashes [block(1) is randomly picked]
then wait 1.5 seconds
then block(2) -> flashes [block(2) is also randomly picked]
then again wait 1.5 seconds
then block(3) -> flashes [block(3) is again randomly picked]
in the meantime as each of the blocks are picked I want to add their UID to the array (that's what the "index" is there for!, each time it adds the UID to array it increases the index of the array by 1)
as I said I'm a beginner at this so please let me know if I misunderstood any of these.