As taught by a MAC senior artist.
Step 1:
Start out by tracing over the tattoo with a nude colored lip pencil. Oak, Stripdown etc. work well. Although this will still be a deeper color it at least somewhat flesh like in color.
Step 2:
With a sponge start to press on Studio Finish concealer or Full Coverage Foundation (they're actually the same product) in an NW shade. Do not blend or rub it on. Press/pack it on. Continue to do this until any black is no longer visible.
Step 3:
With a sponge press Studiofix Powder in a shade that matches the skintone all over the area. Once again, no rubbing or blending, only pressing/packing.
Step 4:
Apply a little bit of the Studiofix powder to the surrounding area. Kidn of feather it out.
It will never be prefect but it will be the best tattoo cover can be.
Another good tip, say you are covering a tattoo on a shoulder blade, apply a little Studiofix to the opposite shoulder in order for the difference in texture to be less obvious.