Recently I purchased a new Merida Crossway, and while the bike itself has been fantastic, I've been having almost non-stop issues with the mechanical disk brakes.
They began a loud squeal not too long ago, and while I did take it back to the shop where I was told they were sanded down and cleaned, the squealing soon began again. Taking matters into my own hands, I have removed them, cleaned both the pads and the disk, and made sure that they were aligned and set properly. While this works temporarily, they then begin to squeal again after only a few minutes of use. Straight after taking it out for a test, I removed the pads once more and found they were covered in this horrid black soot, similar to what I removed before the test.

What's causing this soot? Should I do a more thorough job of cleaning the pads? Will they need to be replaced?
Hopefully someone here can help, I've had nothing but problems with these brakes.