painting a machined rim I have purchased a handbuilt rear wheel that will be used with a disc brake and never with a rim brake (it has a 6 bolt hub). The rim is a mavic a719.

The rim is anodized black but the machined part (rim brake surfact) is silver. Ideally I'd like it to be black for aesthetic purposes and was thinking about giving it a light coat of spray paint.

Is there any reason that this is a bad idea? Is there any other way of easily making it black?

NB: I'm aware that there are non machined rims available. I'm aware that some my see it as ugly or poor etiquette to run a disc hub and machined rim. After a lot of research, I concluded that this was the rim I wanted.
I've had many sets of painted rims.  All of them were painted by a professional whose day job was cars.  I have no idea what the prep and such involved was, but all of them looked great until I cracked the rims, had large rocks take chunks out, whichever.  

Paint is certainly not going to hurt your metal rim, and it may offer it some protection.  Whether or not the paint stays on (which doesn't actually seem to be your question) is a matter involving far too many variables (prep, materials, care, conditions) to answer.  Unless you are planning to use some frightening type of paint I haven't heard of, you won't damage the rim by painting it.