Permanent Blind in Hydrostatic Test

ASME B31.3 piping, hydrostatic pressure test, loose blind flanges ...

You need help. You need to get your insurance company involved, otherwise any pressure test you carry out will need to be repeated for insurance purposes anyway. Your insurance company will need to have as-built drawings, and will send someone to review ghe drawings against the equipment and to witness the test. For those reasons you need to pick up the phone and talk to them about it all.

Asking a bunch of anonymous strangers on the internet is no way to ensure no-one gets hurt or killed.

You put a temporary blind on, pressure test, then you unscrew the temporary blind, rendering the pressure test useless and repeat.

Yes you can use a temporary blind but you will not be able to use the test result.

If this does not make sense then maybe the question does not either.

If in doubt look up you vessel code!

In alignment with that the other posters have said:

(1) consult with your insurance carrier first

(2) IF the vessel does not normally have any normal vent, there has to be some serious consideration as to how you intend to have a liquid pressure hydrostatic test, since you will not be able to get rid of vapor/air space above liquid. Not doing so, will invalidate (typically) any hydrostatic test result, and can produce an air/liquid rocket, if any fitting fails or breaks off. This can be extremely hazardous to workers in the general area.

(3) Normally such a vessel should have pressure safety relief valves, and these cannot be tampered with in any way.

(4)IF there is a normal venting line, you may (and should) have a double blocking valve set on that line, so if you secure these, there should be no reason to introduce a blind flange.

Basically what you proposed in original post/question is intrinsically an unsafe practice and is to be avoided.