First a comment before I get to my favorite damsel fly pattern.
There is a tendency to use a damsel fly and dragon fly interchangeably, but they are different species. Adult dragonflies tend to be thicker bodied than adult damsel flies and their nymphs look completely different from each other.
Damsel fly nymph
The adult blue flies that are commonly seen are blue damsels.
The best damsel pattern I have is the Borger Braided Butt Damsel
Tying instructions are below:
Gary Borger » Braided Butt Damsel
FFF Fly Tying Group Membership Page
Some tiers have used foam to modify the pattern to guarantee a floating fly such as in the pattern below:
I mentioned this to Gary and he asked me whether the tiers had thought he had not considered foam. According to Gary, the trout can concentrate on the drowned damsels and the foam pattern cannot imitate drowned damsels because they always float. So tie the standard pattern first and add a few foam ones if you want. Compare the Borger damsel with other damsel patterns and you will note how realistic the braided butt pattern looks compared to foam, dyed deer hair, or dubbed abdomen patterns. None of these can match the thin abdomen of the natural insect. The Borger damsel is the most popular of all damsel patterns here in the colonies.
And use a strong tippet. The vicious take can often break you off.
Another point is that the stage before the mature blue damsel is the brown teneral phase.
You can use a brown color marker to match the mono to tie up a few teneral patterns. The brown teneral patterns will also match brown dragon flies.
Cortland braided mono comes in 30 and 50 lb strengths. Get the 50 if you can. You can use the braided mono for making braided loops and the 50 lb is stronger.
The videos below show how damsels are vulnerable when they are under the water and how they get trapped in the film. A foam damsel pattern can't get that trapped in the film appearance.