The main use of the black titanium in Fallout 76 is completing the "Mining Wonders" quest, for which you will receive the Excavator power armor. If you have no idea what this is all about, go to the Slag Abyss location, in the southwest corner of which is the Harrakhan miners' headquarters. Interact with one of the signs outside to start the quest. The first stage of the mission is simple: go inside and do whatever you are told, killing all enemies along the way. Eventually, you will reach the stage where you will need to craft the armor.
To craft Excavator Power Armor, you will need a lot of Black Titanium. It is one of the rarest resources in the game, so you are unlikely to find a lot of it during your travels through the Wasteland. Fortunately, there is a reliable way to get this valuable resource. Travel to Junkyard Gulch in the Woodland Region, which lies directly east of Vault 76, north of the river, and where Route 64 splits into two roads.
When you have enough black titanium, return to the miners' headquarters, process all materials and scan the armor to register it. Voila! You have completed the quest "Mining Wonders" and you will be given a Power Armor Blueprint as a reward.
