I have a bushed aluminium bonnet that I will be spraqying with laquer to give the sruface some protection and want to mix in a tiny amount of black paint to get a transparent black effect.

Is that gonna work? Or will it end up looking grey and non transparent?

Or is there a special transparent paint?

Strictly speaking you're supposed to etch prime ali - that said all my body panels are ali & I primed them with either polyester spray filler or 2 pack primer & none of my paints fallen off yet!
As long as the ali has been sanded with something like 800 wet & dry the lacquer should stick ok, but you'll find it almost impossible to get an even colour, it will tend to be lighter & darker in different areas - would suggest experimentation on some scrap first.

There are transparent dyes for adding to clearcoats. They are designed to the put over coarse silvers to give that iridescent candy look.

I havent seen them in black though.

P.S. The is a special self etching clear for use on spun alloy wheels, but I have never seen it for sale anywhere

Have a chat with your paint supplier. Laquer is fine until you get a scratch, take a look at old alloys, they go white once the water gets in. Not a good idea!!!!!