what did the blue roblox nerf gun give item

The blue Roblox Nerf gun likely gave a virtual matching item code for the Roblox avatar version of that blaster, since Roblox-themed Nerf blasters come with a code that unlocks the corresponding in-game item. Roblox support says Nerf blaster codes are found in the package on the code card insert, and the item is then redeemed into your inventory.

What it usually unlocked

For Roblox-themed Nerf blasters, the reward was typically the avatar version of the same blaster, not a random cosmetic. Roblox’s official support explains that these toy codes redeem into the user’s account inventory as a virtual item.

If you meant a specific blaster

Different Roblox Nerf guns had different rewards, so the exact item depends on the exact blue blaster name or product line. If you have the box name, the item can usually be identified from that product’s code description.

Useful note

If the toy was part of the Arsenal line, one example of Roblox/Nerf rewards was the Dart-Warrior skin and Foam Blade 3000, which shows how these promotions sometimes gave game-specific items rather than just a generic accessory.

TL;DR: the blue Roblox Nerf gun most likely gave a redeemable virtual item code for its matching Roblox avatar blaster.