I'd like to explain the standard GR block process, not customized.
-The storage location is optional data on purchase order. (blank is possible)
-You can't input the storage location when you receive GR blocked (mvt 103 or 107).
-You have to specify the storage location when you release GR blocked (mvt 105 or 109).
But the QI stock is different because you have to input the storage location when you receive it. (mvt 101)
Therefore SAP doesn't provide the auto UD functionality for GR blocked stock.
Regards