Mild, Medum and High, carbon content steels can all be blued. However they also each have specific welding techniques and a varying degree of difficulty to weld.
The weld is for a little tab on a rifle magazine. I dont know the thickness of the clip, but it is pretty thin guage. If you've ever held one, you would know. Um...I would be performing a lap joint on it. I don't want to ruin this thing though because I don't have anything to practice on. I have no idea if it is alloy or just plain steel. I kinda feel like an idiot here, but thank you for all of your help.
You really need to measure the wall thickness of the magazine. Then you can figure out your amperage setting (1 amp per .001). The small tab will only have to be tacked in a few spots to work properly. I would use scotch brite and remove the bluing on both pieces, tack weld the tab in place and use touchup bluing and be done with it. Use E70 1/16 filler and you should be good to go.