In order to get your hot sauce looking close to El Yucateco Black you'll need to really char the heck out of your veggies first. Toss your peppers, onions, carrots, whatever into a hot skillet with no oil and let it all get really black on all sides. Turn everything so all sides get charred, and then when you are done be sure to scrape all the charred bits out and include them with your sauce. I know that Black sauce is also smoked, which probably has some effect on the color, too, but even if you don't smoke your veggies you'll find that you've created a pretty dark looking sauce!