CashMoney Crazyboi

Hi, I am having problems with creating Prim Shadow for my store. I drew a square and blurred it to opacity of 48%. When I bring it inworld, it looks very nice, until I look sideways, because I get get my box very thin, there is a lining of black. You know like how shadow will be on 6 sides of the box (since its a square). How can I get rid of that ugly lining?? Thanks