My recommendation is a hitherto-little known gem, called ScreenShade.
There are several decent Mac software screen dimmers out there, of which I've found the ones whose dimming method is to apply an increasingly opaque black 'overlay' over the screen to be the most superior in appearance (providing much higher quality 'low blacks' than Shades which does its dimming via color profile/gamma level tweaking) and in doing so, also playing nice with famous color temperature regulating app, f.lux.
The best examples in this group, are Shady (whose development has been abandoned by its dev and has a no better rogue clone in the Mac App Store), ScreenDimmer, Screen Shade, and Work at Night. (N.b. these last two, confusingly appear to be clones, by the same dev on the Mac App Store.)
But if you're looking to more fully emulate the Mac's in-built hardware dimming in software, you're looking for an app that can have the dimming triggered via hotkeys, system-wide.
Annoyingly, none of the just-listed quality 'overlay method' apps actually provide keyboard shortcuts (at present).
So for all the above criteria AND having keyboard shortcuts, the only one I have found is the humble Softonic-hosted ScreenShade, est. 2008!
It's working on Yosemite 10.10 no problem, and has a decent little preferences window including even customisable support for multiple/secondary monitors! Its only drawback, is that the hotkeys' granularity in dimming up and down is a little less than one would ideally want (i.e. the jumps are fairly large), but this is a small concession given it at least provides hotkeys, and it does go down to very low levels (with fairly good clarity), all the same.
So until contenders like ScreenDimmer, Shady, or Screen Shade / Work at Night do anything about it, if you want something with hotkeys and simultaneous operation with f.lux (and to do it in software), this looks to be only app that can do it.