Does anyone know how to keep the black buttercream piping from fading into a buttercream frosted cake?

One thing is to make the blk as early as possible before using it.  Freshly colored has much more of a chance to bleed. 

You can make your blk by mixing together *ANY and ALL colors* of icing together.  Don't worry what it looks like in the beginning.  

You don't say how much blk you need so here is  general instructions:  Mix together about 2 Tablespoons of red, green, yellow, blue, and brown (just examples of colors you can use!).  Now add about 2 Tablespoons UNsweetened cocoa powder and maybe a teaspoon of HOT water (if it seems really dry).  If it's not dark enough add 1/4 teaspoon brown (or black) gel/paste color.  Do this up to a week up before needed so the color has time to develop and blend.  Use as much color as needed to get the dark shade you need.  If using black, don't keep adding gel color until it is *black!* but just until it is a darkish grey.  It will darken to black when it sits before using *and* even a bit more once exposed to the air when piped  on the cake. 

Thanks for the tips, I will try it on my next cake that I have with black writing!