Interesting !! Have you tried McMaster-Carr, tel:+16308330300. If all you find, is plated, you can remove the plating and reblue, black oxide. I had to do this, on a simulare project.I've tried Fastenal and they only have philips or what not.
No, they have stainless too. But OP wants black or blued.The only black M4 slotted on McMaster-Carr appear to be plastic.
https://www.mcmaster.com/products/c...nt~metric/thread-size~m4/drive-style~slotted/
That's a good one. Thanks.