i don't belive it's necessary on black thread -- where color preserver becomes useful is when you want the color to stand out. it's unless you're wrapping a white blank, your thread tends to take on the color of the blank.
a good way to look at the "downside" of color preserver is that it makes it much easier to remove and replace a guide, should you damage it. i've never had problems with guide wraps unraveling from the use of cp under epoxy.
i've also noticed that you can obtain a nice semi-translucent effect(but still preserve the true thread color) on your wraps by just using a single coat of cp, just enough to barely saturate the thread. 2 or 3 coats will get you that opaque, painted on look. or you can just use NCP thread