If the background was detracting from the portrait than underexposing it away is fine, I think. It wasn't so much that as the "flashy" look the photo had because of the stark contrast between his face and the black background.
Well, let's get this out of the way first: RGB is what your monitor uses to display colours. RGB colour spaces are called
additive colour spaces, because you increase the values of the primary colours to get white. CMYK (cyan, magenta, yellow, black) is a
subtractive colour space, because you subract values from the colours to get white. RGB is used on monitors, CMYK for printing. And important distinction to make and understand.
Second, colour management is a hairy, tricky thing to come to terms with, I find, and I couldn't possibly explain it to you in a post. You need to go out and do some serious reading, but for a quick brief: colour values in an RGB colour space run from 0 to 255. So, a pure, bright red bould be 255 red, 0 green, and 0 blue, say. In sRGB, the range of what
actual colours are displayed along that scale of 0 to 255 is called the gamut. Adobe RGB assigns its 0 for each colour a little wider (oh boy, this is terrible terminology I'm using) in the range of visual colours than sRGB; it's gamut is bigger (note that sRGB does't actually contain all of the volours of the visual spectrum in its gamut, nor does Adobe RGB). The problem is, when you display an image that has an Adobe RGB profile on a device with a gamut smaller than that profile, the colors have to me remapped to values that fit into that smaller gamut so they can be displayed. And that tends to wash-out colours and darken the image as a whole.
Gods, I hope I didn't lose you.
tl;dr: check that Image -> Mode is set to RGB. And if you aren't shooting RAW and intend to display things online primarily, it's worth just shooting sRGB to save time and sweat and tears in the short term. The JPGs above have been assigned the Adobe RGB profile; if you're on a Mac checking this is as easy as looking at the file's info.
*head explodes from trying to explain colour management* I didn't give a very good description; sorry. Best that you try to read-up on it as much as you can. Personally, I don't think you'll get much out of printing in a colour space with a larger gamut, but if you foul-up colour management somewhere in your workflow, some really wacky things can happen later.