Welcome to the forum John.
This black mark is most likely a split/tear/rip in the thin aluminium alloy, notice the very thin edges and the 'corrugated appearance'.
Scale is a big problem as there is not much down there that gives us a clue, half of a clam shell (there's a debate in another thread about clams but, let's keep it out of this thread
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The range of the lighting on the ROV would help if we had accurate information plus, the range of the ROV prop wash. In parts of the footage you can see the prop wash shifting the silt/residue away from the wreckage, if we knew the range of that prop wash it would be a simple calculation then.
Tom and leon. Ric is right in being cautious regarding the ROV footage. It doesn't prove anything! But, at least this expedition will have something to show when it is over. Whatever is down there isn't just coral, although there's a lot of that about, naturally, it's a seamount slope with a reef.
With luck, fingers crossed and everything else crossed, it's the plane we all want it to be, if not, it's someone else's plane, not what we wanted but, something to show for all the effort.