As with any other living thing, the quality of what you are buying depends on the seller. Tubifex are known for being really bad for transferring diseases, as they will live and reproduce quite well in water that will carry numerous diseases that can affect fish.
Commercially farmed and distributed blackworms need much more sanitary conditions, and go through a process after harvesting where they are rinsed, purged of food, which is similar to a fast in fish, given a bit of heat treatment then refrigerated before shipping. This process takes 7-10 days.
If kept properly by the local supplier these pose no risk to fish. If the local supplier is getting them from a local source, often a clean stream or pond, they are not going through this extended process, and the risk of disease transfer increases. Also, if the local supplier is rinsing them with used tank water you have a risk of disease transfer regardless of their source. As long as your suppler is acquiring their blackworms from a commercial blackworm farm, and rinsing them daily with fresh water the risk is as close to zero as it gets. I've been selling blackworms locally for years, get in 25-40 pounds every 10-14 days, and have yet to have a customer have any issues with diseases linked to feeding blackworms I supply.
As far as brine shrimp, I believe the difference in salinity from the brine they are kept in and the lack of salinity in a freshwater tank nearly insures there will be no disease transfer. This is similar to how a saltwater dip for freshwater fish, or a freshwater dip for saltwater fish eliminates many parasites.
To me I believe isn't good enough, I'll contact my blackworm supplier concerning this, as at one time he did sell brine shrimp, and would have a much better idea than I as to any processing that is or should be done, as well as risks concerning disease transfer. I'll get back to you on the brine shrimp question.
