The only bike I have with a black anodized stem is my 1984 Gitane Tour de France.
I just happened to bump into it while shopping around eBay for components. I was also not sure wheter I really like black anodized stems, but this one has a cool pantograph on it that says "Cycles Gitane", so I really had to get it for the bike. In the end it worked out because my headset on the bike is a black anodized Stronglight A9 (Not the original B10) my Stronglight 107 big chainring is also black anodized and my Chain is a Sedis/Sachs black and silver linked chain. The bubble font Gitane graphics is also black with yellow bordering and my RD is a silver and black Simplex LJ 4000 so there's certainly enough black stuff on the bike to match the stem.
one thing I had noticed though, when shopping around for stems is, generally, black anodized stems are cheaper than silver clear anodized ones.

To each their own but I don’t think the seatpost and stem need match. However, I think it’s important that when using a black stem the handlebar tape does _not_ match…