Royal Icing Outlines Cracking

It sounds like it's too dry/drying faster than you're used to.

Have you tried adding a teaspoon of glycerin or corn syrup to your royal icing? That might help...

Not sure, as it sounds like the icing you use already has softening agents in it. Though I don't work with meringue powder-based icings and I've never had this problem; but then again, my environment is usually more humid than dry except in the dead of winter. All I can suggest is to lay other icing next to the outlines sooner than you are doing as this may keep them from cracking and popping off. Or run a humidifier. 

I'm not sure why only your black icing would be affected either. Are other colors drying and cracking the same way? It might be useful to do that test if you haven't already - perhaps there's something weird with that batch of color, though I'm skeptical.

So it's just the black icing that cracks and not other colors? When does it crack - while waiting for flood icing? Or even after being flooded?

Most of the time black cracks after it is completely dry. My line work. I use one consistency to line and flood and have had no issue with cracking there.

I can take my hand across the top of the cookie and brush my line work away like it was never really attached.

Julia, you would be a great detective! You have me thinking now.  Is my line detail line work suppose to have a flat side where it is attached to the cookie? Could it be that my line work is too rounded and not enough is touching the surface of the cookie? I tend to make my dark colors thicker because they get very runny.

Well, I don't know about the detective part; I don't think I've helped with this issue very much . . . but the thicker you mix any icing to start, the faster it will dry and the more likely it will be to pop off. It's the spontaneous cracking part that I don't understand - does it just crack as it sits there drying, without you moving the cookie? Or only when you lift and move the cookie? Is cracking minimized if you flood sooner?

I am not sure about Ohhercraftiness. My issue is when it cracks and breaks off, after the icing is completely dry. If I accidentally brush against it or even when I am packaging it by itself in a poly bag or on a platter! It is the  the finishing line work details on top above the flooded area of the cookies.

Yeah, that can happen. Tight/thick icing piped on dry icing comes off very easily. Maybe you need an alternate packing strategy or to lay on the detail icing when the underlying icing is more tacky?