Dermablend works well to cover lots of things, but is definitely not transfer or sweat proof. The key to these products is several thin layers. Trying to glob a bunch on too fast is the wrong approach. So, you have to apply a SMALL amount and blend blend blend blend and repeat that process until it's covered. Also, make sure you shave the area to be covered as makeup will go on better with no hair (even if it's not a place you think is hairy, shave anyway. Peach fuzz will make the finish less smooth). Another thing is that having someone else do it for you is helpful, so take your MOH or a BM or mom with you to the makeup counter and have the lady there help both of you learn how to apply so someone can help you.
Like I said in my earlier post, most of these things don't work that great. Sure, with enough layers, they will cover a tattoo, but it's never going to blend completely with your skin tone and look natural. Most of the time your skin is still left looking like it's been "patched". I still say airbrushing is the best option.