Should unsanded grout become darker when wet?
Very nice looking job Jared, congratulations!  It is very normal for unsealed grout to change to a darker color when wet. This will happen even with fully cured grout.  Feel free to buff any excess grout or haze off your tiles, it won't hurt anything and that also is normal for newly installed tile.  If the darker color when wet bothers you, then go ahead and put a couple of coats of grout sealer on after about two weeks from the original grout date. Even though the manufacture claims stain resistance, it does not claim to stop the temporary color shift when wet.  Sealing the surface of the grout will stop the wetting of the grout and end the color changes.  You may find you need to reapply some sealer every year or so in a shower surround. Apply the sealer with a small foam brush just to the grout,  Buff excess off the tiles 15 to 20 mins later.  Looks Great!!!   Good Luck.