I used Shooter Solutions Rugged Gun Blue to replicate the black finish on fasteners but not sure it's available anymore (web site says it is temporarily down). It came in a kit with the solution and an oil. Like Dataway's example, there are other inexpensive "cold bluing" products readily available online (Brownell's, Blue Wonder, Van's) but I haven't used them.
https://www.ktgunsmith.com/rugged_gun_blue.htm
If your fasteners are rusty, overnight in Evapo-Rust will work miracles and is very safe.
I did NOT bead blast but would typically soak the fasteners in a concentrated solution of Drano overnight to clean. Obviously, wear gloves and some eye goggles..
If your fasteners are plated, you'll need to strip the plating first... these cold bluing products only work on bare steel.
The bluing (blackening) process was simple, fast, and delivered a very uniform and black finish.
The oil in the Shooter Solution kit was some type of "air hardening" or "drying oil" that left a heavy film. You could use Linseed or Tung oil which both harden to a varnish that will be more durable than a gun oil or similar lubricant.