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I’ve just purchased a new Trek Roscoe 7 (aluminum hardtail mountain bike) recently and I’ve noticed this tiny little dent today. It is on the drive-side chainstay near the bottom bracket. 
Should I be concerned about this dent? Is this a paint dent or a frame dent? Since it’s on a high-load area of the frame, will it affect the frame’s structural integrity and strength? What could be the cause of this dent and what should I do about it?
No. Stop worrying. Go and ride your bike and put some real dents in it.
Without sanding it away it’s hard to tell if it’s a paint bubble or a small void in the forged section of the frame. Either way it’s of zero consequence for the structural integrity.
EDIT
It’s unlikely to be corrosion in a brand new frame. You’d also need a local marine environment to have any real issues with that.
However if/when you get aluminium corrosion you can sand the paint and oxide away and leave the metal to open air.
You have a very strong/overbuilt production line frame. It should last you at least 10 years excluding crash damage.
