Paint

Make a to-scale sketch of the whole wall space, the side board and television. Consider massing framed art on the wall around the television, but you don't have to keep the TV in the center. Put it on the side. Use the space at the ends of sideboard for tall things---floor lamps, plants, sculptures. TALL vases, tall flowers---any thing that draws the eye upward.

A single thing in red is a nice visual counterpoint to all that black---but only one red thing. It could be a single silk hibiscus tucked in beside the TV. You have a great "canvas"---I'd love to work with it. Have fun!

I've always love a gallery wall of your favorite art or pictures of friends and family in frames of different sizes. I would opt to go for the same color frames so it's not too over-bearing. This will remove the TV as a focal point and your guest as well as your self will enjoy viewing the art/pictures.
I do like Sunnyblue's idea as well. If you go her route, start searching garage sales, flea markets or craigslist for those perfect accessory pieces that will compliment the rest of the room. I too would love to join SunnyBlue in helping you decorate this space.
Good Luck but most of all, have fun with this.

I think a nice wallpaper would work - something bold, slightly retro (to match the wood of the credenza). Total feature wall.

Thank you for the comments. I look forward to amassing my art collection for this wall!

I was going to say to put the television on the wall, and then you can put some cool things on the credenza. First, though, you must paint.

I would paint it a bright color and when styling think threes!

I encountered the same dilemma in an old apartment... We settled on wall decals behind the plasma tv in a large formation taking up nearly the whole wall!