Hey varomix -
The quickest and easiest way is to create a Material Parameter Collection and add whatever vector or scalar varaibles you need to that collection, then in the PP Material which you are using as your Blendable add the parameters through the Parameter Collection node. Then in your Blueprint (I tested on Level Blueprint) use the Set Scalar or Vector Parameter node that references the Parameter Collection.
Let me know if you need further explanation -
Eric Ketchum