I'd vote against the blinking light. I really don't like them.
I've only ever seen them on new cars, I've never seen a blinking tail light on a ricer.
As far as legality, it might be something that's legal from the factory but illegal to do aftermarket. Tint is that way in Georgia. If you take your car to get tinted, the legal limit is 35%. But if you get your vehicle from the factory with "privacy glass" or whatever, there's no limit to how dark the tint can be