Here's a way to get rid of your blood worms: You will have to take all of your fish out of your pond though...
Also, depending what your filter is, you would want to take it off, or somehow block it off from the water in the tank.
Get yourself some Potassium Permanganate. (since there are no fish, you can add a large amount)- this will definitely kill your blood worms.
the water will be purple, and once it's worn off, the water will turn brown- You will want to heavily treat it (9 grams does 1500 gallons) and then for your filter (if they are in it) run some high salinity salt water through it.
If the water is still purple when you wish to add the fish, dump in 1 cup of hydrogen peroxide, get your air pumps going, and then you can add the fish.