I'm in the process of making black for the first time too. I read here that you start by using brown, let it set, and then add black, let it set, add more, let it set..............etc. etc. Some said to let it set for hours and some say overnight. As it sets it gets darker.
I will see tomorrow how mine turns out. I've added the brown and black once so far. 
Susie
For black - I have had better success with powdered color over gel paste because it seems to take advantage of the tiny amount of liquid in the bc and stays really really black. I've also added black gel paste to dk.choc. fondant and had acceptable results. Adding gel paste to bc has been a losing battle of really dk gray. 
I personally just use chefmaster in black. It comes in a squeeze bottle so need to mess with toothpicks. just let it set a bit and it gets really dark. You can see on my Marvin cake how dark the black is.
and yes, you can start by adding cocoa as well.
April,
I've found that starting with chocolate icing is more successful. It takes quite a bit of brown to get the icing brown enough before adding the black to get a "true" black and not a dark steely gray. I start with a chocolate buttercream and then add black - works every time.
Let us know how your cake turns out.
Liz
For me to get a good black color - I start 1 1/2 - 2 days ahead of time. Start with brown color and some black and let it sit. Black and red do take time to deepen. Now if you use wilton, I notice it never works for me. Americolor rocks in color and does not bleed for me.
For black - I have had better success with powdered color over gel paste because it seems to take advantage of the tiny amount of liquid in the bc and stays really really black. I've also added black gel paste to dk.choc. fondant and had acceptable results. Adding gel paste to bc has been a losing battle of really dk gray.
I TOTALLY AGREE. Honestly, it's a waste of time for me to do it any other way. I just use the powdered color and be done. Plus, it doesn't have a bitter taste AND it gets darker the more you let it sit. I buy a container and it lasts me a long time. I use powdered for all of my colors and gels for my fondant (if I have to). I'm the biggest fan of powdered and I'd suggest it to anyone! JMHO...I rarely buy gel especially if I can find the fondant already colored.
Please try powdered. I can't imagine waiting days for some icing to turn black. Not even hours. If anyone needs a pic to see how black my black gets, just let me know. And I've never had a complaint.
This is how I've done it......first color your bc a dark base color (green, royal blue, brown, etc) and make sure that your base color is very very dark, then I add the black, (lots of both colors, I threw the toothpick out ages ago). I use the chefmaster gel pastes that you can get from phiel & holing. Getting back to what we were talking about..... I reiterate, dye your icing the base color first and THEN add your black. Let the color bloom for a while to make sure that it's truly black (if you need to add more do so by all means).
http://www.cookiesinseattle.com/product.php?id=5077
If you're into Chefmaster products - I have gotten from here before...
http://www.prettypartyplace.com/cm-4301.html
Again - a little goes a LONG way
Satin Ice offers tons of colors - I use intotheoven.com - but many places sell it. It tastes good and yes, just use hmbc underneath (or apricot glaze).
Yep, yep, yep...once again. I agree. Kpcrash is on top of it! 
I buy my powdered colors here but they do have a website and they ship. www.bakinstuf.com. THE nicest people in the world!! I love them and they are extremely helpful. Been in business for years (since I was a little girl). I'm not sure of other places to buy this 'cause I've always used them. I use CK products...Candy n Cake powdered food coloring. They suggested this to me when I was racking up on those tubes of black icing because I couldn't get it black enough. And I make my own buttercream as well so you're good to go 
Again - a little goes a LONG way
DITTO!
BTW, they offer 2 different sizes also. 1 for $1.99 for 3 grams and the other for $4.99 for 18 grams. I usually buy the larger of colors I use alot like black. Hope this helps....Kpcrash has also been very helpful. I would try those other places as well.
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