u're on d right track, when the Hydrogen gas is excited, they gain energy. Now, when the return back to their groundstate(or initial position) they release this excess enrgy gained in form of PHOTONS(light). This is the light that is observed as emission spectra. This emission been distinctive to individual elements. So, there shouldn't be any light(visible) presesnt, as u know it is observed(spectrum) in a dark background.
Hope this helps