I stained the top of this nightstand black. Clear coated it with a spray matte clear and it turned out like this. I used a 1000 grit sandpaper and gently sanded the top. It seemed to just make the blotches pop more so. What can or should I do?
Blotchy stain is the result of wood not absorbing stain evenly. Pine is notorious for this, but many softwoods have this problem. The sap has resins in it. This isn't even through the wood, so places where there is more sap take up less colour. This can also happen due to uneven sanding.
Do a precoat with some material to make all the wood somewhat less absorbent. Shellac diluted with alcohol is often used for this.
Wet the surface first, then apply your stain (water based stains) or wet with solvent for oil based stains.
Some woods with an open grain need a filler first.
I got good results on pine by mixing water soluble stain with water soluble varnish, then applying a zillion coats of it. The colour with each thin layer got deeper and richer.
In general: When you do a project, keep your scraps. Sand them the way you do your project, and use the scraps for testing your finishing process. This avoids a whole lot of pain when you find something tht doesn't work.