I've  wanting to dye some plastic parts to black. I believe I've read were some people dyied their HK rifle furniture, those Gray ones, to black using dye packs in boiling water. Is this true or am I smoking something?

Vinyl dye, get it at auto parts/paint store.

Thats the spray can stuff right? will that scratch off or anything?

Rit dye in a pot of very hot water.
If it's already black, it's kind of hard to do.

The vinyl dye "soaks" into the plastic.  If you scratch it, you basically scratch the plastic that's been colored, and reveal the "fresh" plastic underneith.  The dye though, does not scratch off, and isn't really considered a coating, rather its bonded to the plastic.

Quoted: The vinyl dye "soaks" into the plastic.  If you scratch it, you basically scratch the plastic that's been colored, and reveal the "fresh" plastic underneith.  The dye though, does not scratch off, and isn't really considered a coating, rather its bonded to the plastic.

You are talking about the RIT dye right? I'm not dying black to black but gray to black.

If it is Nylon, it can be dyed.  It is best to dry the stocks by "baking" them at 120F in you car on a hot summer for a few days.  Nylon absorbs water, releasing it slowly at elevated temperatures.  

It then takes hot water or high pressure to get the water-based dye into the Nylon.  Hot water is better because you can inspect the process.  Have a few boxes of dye handy to get the desired shade, no two parts are ever the same unless you know they are the same lot and same age/treatment.

Only new stocks because oil etc will effect results.