I am not sure if a reservation can really block the item, this often just depends on other (e.g. ATP) settings in your system. You did as well not mention if you are able to separate the stock from the other stock in SAP e.g. by batch number
If you do not have batch numbers and all stock is just in one location, then you just have big number of quantity but cannot see from SAP which part is actually reserved and for whom.
At least you would need to support this "reservation" with marking the items physically in the warehouse.
Instead of a reservation you could as well create a sales order to allocate the stock portion.