I don't know Blockly.
But Scratch will become the reference soft in all french school next year.
The real-life interface is more easy for pupils.
I find a solution :
I use a picaxe 08M2 between Picaxe 14M2 and servo moteur.
Pupils use Picaxe 14M2 and they can switch output high or low.
When the output is high, Picaxe 08M2 drive servo motor.