Which volume developer to use to make a blonde go black?

I've been dyeing my hair black for years and years, but I never stopped to think if I was doing it properly, and I've never bothered to ask. I've been using a professional, permanent Clairol black, and usually I use a 30 or 40 volume creme developer. Now, I'm no stylist, so I know jack about developers and all that. Is that too high? My hair never seems to suffer, but I don't want to damage it either. I just want my hair to STAY BLACK haha. Blonde roots get so tiresome.