Blueprints

allows you to get an upgrade that copies other upgrades through that MOTHER FUCKING SON OF A BI- Roachard.

Give him an Atlas Oyster (20% chance to acquire from dismantling any skin), and he will give you a Blueprint upgrade for one of your weapons, grenades or the player upgrade.

What mods does this support?

Mostly, any upgrade that has a stacking buff/debuff. Look for "+" or "-" signs in the attributes section of your upgrade.