Oh please don't give up! Dahlias are so beautiful. I'm sure someone will see your post and rescue you. My first suggestion is that you spray with orthene which is a systemic and they tell me Bonide makes a good granular systemic. Sometimes hard to match the insecticide with the critter. Steve in Baltimore County.

I would encourage you to try again (either with your current tubers or get new ones, whatever you choose). I'm just finishing my second season of growing dahlia too and I was just happy to see them bloom at all. If you have a pest problem again, either take a sample (plant is good, insect is better) in to a garden center or your local extension office to have it identified or take pictures and post them on here.
How big was the "beetle" like critter?
And the "blister bug" you referred to I would imagine was some kind of scale which can be hard to control because they protect themselves so well. Common controls are dormant oils which smother them more or less.
From what I have read, spider mites are a common dahlia pest, and they thrive in hot, dry conditions. Look for the tiny mites on the undersides of leaves and they will produce a fine webbing and stippling (spotty yellowing) on the leaves.

It was also my second year. My first year I didn't read anything, just put them in the ground. They were spectacular. Than I had a whole winter to read everything about dahlias. I started doing the pinching at the 3rd set of leaves. That did me in. I would get about 4 to 6 long stalks with flowers on the very top. So discouoraging after the first year. I also had pests. Whitefly was one and it was awful. I used everything that was suggested. I jut hope they don't come back next year. Try some in containers were you can keep an eye on them.

Do read up this winter on the use of horticultural spray oils. Steve in Baltimore County.