I have the same problem... I have made a 3lb/gallon batch of mead with only spices and a FULL orange.
When I read "Full orange" I thought it meant the entire orange, thrown in. So I did that, and poke some holes in it. Orange peels are bitter...
Anyway, I've heard that once a batch is bitter it can take up to a year (or even more) for it to go away. Mine is only about 45 days old, so I have a lot of waiting to do.
The thing I CAN say about yours though is many berries, unless completely broken down and mushy, contain a lot of tannins. As the brew ferments (i.e. loses sugar) the tannins remain constant. So what you're tasting is the absence of all the sugars and the uncovering of all the tannins.
However this cannot possibly account for the total bitterness of yours (and mine) so I'd like to know what process yeast (or perhaps a contamination) does to make something bitter. . .
And how the heck does it dissipate with time...?