Unless your printer specifically states they want CMYK, they probably want an RGB profile of some sort. I believe a lot of inkjets use CMYK, but I'm no expert. Places like Luminous-Landscape have a good number of folks who print at home and can begin to explain the intricacies of a colour-managed workflow for home.
'Dark' pictures are tough to get right, and it takes a little experience to learn to 'see' what an on-screen image will look like when printed out. There are a few issues at play:
- Monitor calibration. This is primarily about colour accuracy and reproduction, and will be very difficult with a cheaper (non-IPS) display panel which has colors and brightness that change significantly depending on how you angle the monitor/move your head. The luminance (brightness setting) also plays a major part - on my older Apple Cinema Display I have to turn brightness down to about 1/3 of maximum in a room with soft, neutral light (no direct sunlight in the room, shades drawn, lights on).
- Color space. Google 'color management' and prepare for some fun. Most print places and all web outlets seem to prefer sRGB profiled images. The better print houses will have suitable ICC profiles (profiles that describe how devices interpret colors) available for the specific printers and papers they use, allowing you to 'soft proof' what your images will look like.
- Print media. Each set of media/inks/etc. has it's own available tonal range. For 'dark' images, the difference between a backlit monitor, even one that looks dull, drab, and dark (= what my calibrated monitor looks like when I'm editing images) is huge, and the best 'dark' prints are ones that look excessively bright to me on the computer monitor. Some images look best behind acrylic, others look good under matte mounting, some better under glass, some best on pearl or metallic papers (sunsets look great on Fuji pearl, for example).
- The lab. I want a lab that does not do ANY post processing on my images, which many do - I want to be in control of color, brightness, sharpening and so forth. The better labs understand workflows and colour management, and if you do as well, it's easier to pinpoint problem areas. You can also ask them whether they think a certain image will come out with reasonable amounts of detail in the dark areas. As an alternative print full-size crops of smaller sections before committing to a large panel of anything.
I love prints, but I don't want the headaches and costs of large format home printers. Besides, I usually want really big prints (more than A3), so I prefer to pay a good printing place rather than messing around with home printing.