AFAIK skin prick tests are the best for contact allergies, blood tests unlikely to find anything but severe allergies.

and I'm not sure how to find mild food allergies directly, seems to be all food diaries and correlation which feels mostly useless..

There also seems to be a hair test that AFAI can tell is an outright scam.

The problem with blood tests in my experience is all the false positives, skin prick tests are often used based on result from blood tests. But when having many allergies it is normal to only check for the most severe using skin prick tests.

Food diaries are probably good for those that are more experimental in what they eat, the difficult part come when it is a combination of things that give a reaction. The most important is to learn to listen to your body and not buy into peer pressure when someone urge you to try something new that you are not sure about