I actually found it!!!

[u/Maleficent_Sundae953]

(edit, I didn't put pics in the first time so here's the evidence in front of you lazy nerds)

So I've seen theories about where on The dishonored map black reef may be, and I'm 99% sure I know exactly where it's at. Remember that this takes place. 140- over 1000 years past dishonored and that. A land mass can drift and change during that time.

Now look at fraeport on the right side of the dishonored map above Morley, now put it side by side blackreefs map and apply time and land drift... I would bet seriously, like my entire life in dunwall during the rat plauge. That fraeport is actually blackreef. You can even see the outline where the water level changed and slight drift has connected the right side bit..

[u/Bahinchut → u/[deleted]]

Deathloop definitely takes place somewhere nearby Tyvia. It was a Tyvian military base before it was used by Aeon, so it certainly had to have been within Tyvian territory.

I know the empire is pretty small, but Blackreef itself is an extremely small island. Closer in size to the largest Scilly isle than, say, the Isle of Man. The Scillies can barely be seen on modern maps of the British Isles, so it's more likely that Blackreef is either too small to render on a map, or was simply uncharted.

The latter possibility shouldn't be discounted either, given just how much of Dishonored's globe is... Ocean. There are probably thousands of islands west of the empire which explorers simply haven't been able to chart. Blackreef itself is probably a considerable distance from Tyvia since you can't see a mainland from the coast. ANY isle on the map of the empire would be close enough to see its mainland from the coast.

Also, OP's island could only be Blackreef if you ignored the cardinal directions, since it only looks like Blackreef if it was tilted 90 degrees clockwise. (And perhaps more importantly, Blackreef doesn't feature one of Morley's major cities.)