Personally, I think BLU weapons are fine as is. You get the story progression weapon, and the challenge mode weapon (which is the typically the fancier weapon with effects).

Rather than a Relic, if BLU were to have more weapons added for the sake of glamour options, then I think they should be Achievement Rewards for undertaking certain activities with all BLU parties. Kind of like what's already been established for the Malboro Mount.

OR a cheeky alternative, since Basic Instinct is a thing... maybe achievements earned by completing certain activities on BLU solo...