The blinding layer considers important for a couple of reasons:
the first reason is it uses as a leveling layer.
The second reason is to maintain the mortar of the concrete
as well as it is useful to protect the concrete body of the foundation from water and the salt attacks as well.
There is no optimum thickness for the blinding because as I wrote it uses for leveling. Anyway, if it exceeds 10-12 cm, the shrinkage reinforcement must be taken in the considerations.