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Alright, so, I need some help with my Deathwatch marines.

Y'see, I'm painting up a 5-Man squad of Sternguard for Kill Team at a tournament in a couple of months (Coastal Assault 2014 if you're interested) and I'm nearly ready to paint them after a couple more things I need to put together.

Now, I know the entire left arm is gunmetal grey and the right shoulder pad is usually the Marine's chapter, but I want your input on the black. I'm basecoating with Imperial Primer and doing a base layer of Abaddon Black.

What washes should I use? I'm thinking Badab Black, but I want to try something different. Any suggestions?

Black is almost never truly black. It is a very "blackish" dark version of a chosen colour or - if you'd rather - it's black layered with a colour+black mix enough to seem black without being really black.
A random example on the spot (with vallejo colours, but the principle stays true with gw and other brands):
Black primer (or primer and black basecoat)
Vmc black grey (or mix 1:1 black and a dark grey), leaving only the deepest recesses
Vmc dark grey highlights (with or without a black wash, in order to darken/deepen the shadows)
Vmc light bluegrey (or light turquoise) extreme highlight

I use Abaddon black with Dawnstone highlights with Ulthuan grey extreme highlights to do my black armour.

As said above, your probably better of base-coating in grey, then washing in black.
Putting black wash over black paint over black primer... is just going to be 3 layers of black, first matt, then satin then matt again :/ caking up the details while defining nothing

I did the same thing with my Death Company, prime dark grey, add some grey highlights, wash with Nuln Oil.