Blueprints lost?

[Lemon Senpai]

I used a blueprints from a mythic chest a while back when I was new and trying to figure this all out but. I want to build this weapon and am realizing that the blueprints are no longer obtainable from that chest. Is there any way to get them?

[Xaelon]

Blueprints are a permanent account unlock. Once an item is unlocked it's unlocked for good.

Mystic chests are also one time use.

Did you use the BP frags you got earlier to increase the rank of a different blueprint? If so, then they're gone. You'll have to either make a new character or loot that chest again next season.