I need black fondant but only had regular white on hand. I added black gel food color. By the time I rolled it out and put it on my cake it seemed more blackish gray than true black. Any ideas on if this is due to not having added enough color, or maybe using too much powdered sugar to roll it out?

Have you let it sit for a while? Sometimes the color will darken over time. I dye mine and then let it sit over night. Sometimes you end up having to put more color in it the next morning. The best bet is to purchase black and red fondant... I always keep satin ice black and red on hand...just takes too much color and time to get it the right shade.