Re: I should have bought an Epsom but...
I have an HP Photosmart D7260 which uses different cartridges than your printer, but the HP website calls both ink sets Vivera so they may use the same inks. I get pretty good blacks. Different printers have different drivers, but mine has a tab in the Preferences box marked advanced. In Advanced there is a setting for ink amount which can be changed to increase the ink laid down. There is also a setting to increase drying time which might be a good idea if you are increasing the ink.
If you are using black to print a negative and can't get paper white in your image, try printing in red (negative open in Photoshop, new fill layer, solid color, R 255 G 0 B 0. Mode screen.)
Hope this helps. Charles Portland Oregon