You can actually just follow the Awakening/Berserk Potion trick, which set the equip level to 50, then only level 50 and above only able to use the items.
603,Old_Blue_Box,Old Blue Box,2,10000,,200,,,,,0xFFFFFFFF,7,2,,,50,,,{ getrandgroupitem(IG_BlueBox),1; },{},{}
Your attempt above are workable, but it will remove your item if you character have less than 50 level.