A blood test wont really show up what your dog is allergic too.

You can be asked to be seen by a dematological specialist. I used to work at a referral centre, so saw a lot of skin complaints. They are amongst the hardest conditions to diagnose and treat.

You can have a intradermal test done. This involves shaving a large patch on the side of your dog, and injecting very small amounts of things that dogs are commonly allergic too. The site is monitored for reactions. Based on the results, the vets are able to have vaccines/treatments made specifically for your dog (the ones where i worked were shipped over from denmark) and can be very successful.

This is very expensive though. The dog cant be on any treatment at the time of the testing either.

Its just a thought to let you know what options are avaiable.