Might be a little late but...
I use a subevent to
check for floor(random(50)) = 1 then spawn my special block.
Else if floor(random(100)) = 1 then spawn my really special block.
Else spawn a random normal block.
The random number is the 1 in x odds of picking it.
My first two colors are the specials so when assigning a random normal block color i use set Color to floor(random(3,numTiles+1))