Seat belt wont let go

Test as its presented,
I wouldn't attempt to unfasten the black buckle (no requirement), with the seat belt fastened on the other buckle will give the opportunity to check the overall operation of seat belt and both locking mechinisem when pulled.
Advise that the seat belt is presented with black buckle a attached in my opinion.

The black buckle is actually an anchor point, no requirement for the anchor point to release so if the seat belt functions as required it's a straight Pass.

You'd only be advising the customer about something they most likely already know. Save your ink and time...

Is it a seat belt buckle ?

Does it release ?

Does the Manual make a distinction between black & red ?

Food for thought !

Some of the French stuff have what look like buckles as permanent fixings alongside the front seats. I don't consider those as buckles, and in the rear of Scenics and similar I have seen spare buckles which have no belt with the seating layout in use at the time. As far as I am concerned it's a case of "has it got a belt, is it in good condition, could I strap myself in and let myself out? Move on"

Personally, I would examine it like any other seat belt buckle.

fail for me, The locking mechanism of a seat belt does not release as intended

I agree David
The chances are just to add , that the black buckle may need another action to release it eg a key then pressed.
We all know why we need seat belts.
Why do they have to release under tension when pulled?
Well the answer to that is so the person restrained by it can be released, the belt in question only needs a locking mechanism that can secure and release that being the red one which does work.
The black buckle will not prevent the person restrained by it to get out of the vehicle, the belt can be released ,so why fail it?
Or to put it another way , does the belt lock as per the im,so it restrains the occupant-yes
Does the belt release so the occupant is not trapped- yes ,therefore its met the minimum requirement.