trimming, coloring, & henna Q

I have been thicking about dying my chocoalte brown hair to black. It's elbow lenght and in good condition. If I were to do this at home would one application box be enough? This doesn't seem like a radical change and I thought it might be safe/easy to do at home. I have never had my hair chemically treated in any way and am a bit nervous. Am I underestimating the complexity of this procedure - if so I'm off to the salon. Does any one one have a recommendation about trimming my hair either right before or right after coloring. It's time for my regular trim now.On the instruction guide for a home hair coloring kit I read that the product should not be used by anyone who has used henna in their hair. Not being a henna user, I was curious as to why that it is and the possible effect. Does that mean, until any hair that was touched by henna grows out and is cut- or just a few months to give it a chance to wear out of the hair?