ok so interesting Question

[Bad Boy]

Use weapon fire or weapon acquired target as the on trigger. Using weapon lost target as the off trigger might have slightly spazzy results as the ship fades and unfades whenever it sees an enemy. A delay could possibly work to offset that though.
This would however result in your ship's fading be dependent on one weapon, so I'd recommend giving that weapon slightly more range than all the others on the ship and high turning so it doesn't lose targets easily. Even so, I'm not sure how well it'd work.
Also, keep in mind that you can't fade the core. Assuming you want an invisible ship, you could try colouring it almost black, though that'd leave a the circle bit showing and would stand out noticeably from the background. Or you could place an almost black, non-fading section on top of it. This'd still stand out, but less so. Alternatively, if you can cook up a sprite the BSF background, it could possibly work as a core cover and stand out less.