What you're describing is black asphaltic adhesive, and it's often called "black out". Whomever told you to sand it off wasn't familiar with the stuff, as sanding it is just going to plug up your sandpaper.
What you should do is this:
Phone any carpet retailer and ask to speak to the Installations Manager.
Ask the Installations Manager who sells flooring installation supplies in your area.
Phone up that company and ask if they sell Oil Flo or Oil Flo 141 made by Titan Labs.
If they don't stock it, they probably know who would in your area. Or, just Google Oil Flo 141, and you can probably buy it online from lots of different places.
From what I understand, Oil Flo 141 is supposedly better at removing black asphaltic adhesive than Oil Flo, but you might want to confirm that with Titan.
I've used Oil Flo to remove black asphaltic adhesive from concrete, and I also used Goof Off and a mixture of detergents. That's cuz from what I could see, Oil Flo seemed to me to be just a mixture of a solvent and a detergent. You simply use the Oil Flo or Oil Flo 141 to dissolve the black adhesive, and then clean up the black mess with water.
I did the same thing as follows:
1. I used a heavier petroleum distillate (Goof Off or Goo Gone, I forget which) to dissolve the black asphaltic adhesive. Scrub with a scrub brush to ensure all the black adhesive is dissolved in the petroleum distillates.
2. Once you have a black mess on the floor, add a detergent. In may case, I used a 50/50 mix of Simple Green and Mr. Clean. That seemed to work better than either alone. When you add detergent to the black mess, the detergent will dissolve in the black mess. That's because the hydrophobic (water hating) ends of the detergent molecules are attracted to and will dissolve in the petroleum distillates. The hydrophillic (water loving) ends of the detergent molecules will simply have no where to go.
Scrub with a scrub brush to ensure that the detergent is thoroughly dispersed in the black mess.
3. Now, add water and the black mess should turn into a grey mess. The colour change happens as an emulsion is formed. An emulsion is one liquid suspended as tiny drops in another. When you add water, then the hydrophillic ends of the detergent molecules all want to dissolve in water, and the hydrophobic ends of the detergent molecules want to remain dissolved in the petroleum distillates. The result is that you get gazillions of tiny droplets of petroleum distillates suspended in water, and right at the interface between the two will be a film of detergent with all of the detergent molecules oriented with their water hating ends pointed inward and their water loving ends pointed outward. The black adhesive will be dissolved in the tiny droplets of petroleum distillates suspended in water.
The colour change from black to grey occurs because by forming an emulsion, you now have two distinct phases instead of one. You have the petroleum distillate liquid suspended in water, and light will reflect and refract at the interface between the two liquids. It's this reflection and refraction of light that makes snow banks, clouds, waterfalls and the head on a beer white in colour, even though nothing inside any of these things is actually white in colour. Milk is also white because it consists of tiny droplets of fat suspended in water. The more droplets of fat, the more reflection and refraction of light goes on inside the milk. And, this is also why skim milk is so poor at whitening coffee. It doesn't have nearly as many of those droplets of fat in it, so it's not nearly as good at reflecting and refracting light, and therefore is less opaque than regular milk or cream.
And of course, whenever you mix two colours, you get something in between. If you mix black and white, you get gray. That is why the black mess turns into a grey mess when you add water.
4. Because the petroleum distillates are now suspended as tiny droplets in water, you can now clean them off the floor with a sponge and bucket. That sponge will get black and covered with oil as tiny droplets of petroleum distillates break on it's surface. So, buy a few spare sponges... you'll need 'em.
In my case, I was able to remove the black asphaltic adhesive from concrete with Goo Gone and detergents, but I have to say that I did get better results using Oil Flo. I've never used Oil Flo 141, tho.
Hope this helps.