I think this was in referrence to adding "reset.css"[1] as your first css file. It is very "black box," as it is something that you add but never modify.

[1] http://meyerweb.com/eric/tools/css/reset/

Wellll.... Meyer says right there "In other words, this is a starting point, not a self-contained black box of no-touchiness.".

I understand a lot of people treat it like that, though. I modify the reset to set defaults for each layout myself.

If you need to minimize it, run it thru Sass as the final step before publication. Sass will blackbox the living hell out of it. But, you know, in a way that makes sense.