Try Navy blue

I like Brandi's idea of navy blue, and would add that you need to get a dark blue that has the same underlying tone as the blue you currently have so they work together. The easiest way to do this is get a bunch of blue paint samples next time you're at a big box store [I like Behr's samples as they show them with the different shades of the same underlying tone] to figure out what blue you have. When you find it, you can go a shade, or better two darker for your accent wall.