The posture is that of a grey heron and the colouring closest to a 1st year juvenile, although obviously still far too dark. The legs seem a bit darker than normal and (to me at least) rather short. Has anymore been seen of this bird over the past month? The records described here suggest an elusive bird who likes to hide. The note about the bird pointing its bill upwars also indicates a wary animal: this is typical bittern behaviour, trying to disappear in the reeds.
The notes about a (different) dark heron in the area last year suggest this bird could be its offspring. If so, a hybrid is possible, as an abnormal animal may not do well in competing for mates and so plausibly has a higher chance of hybridizing. Bittern comes to mind. There is a story about an imported group of dark-morph reed egrets being released n Germany a long time ago, and this could play a role in dark hybrids.