They are going to look fairly dark without the backlighting on - that's just a side-effect of the design.
But soaking them in warm water with a couple of denture tabs dissolved should clean them up nicely.
I tend to use 3 denture tabs for a full set of keycaps. Put the tabs in and wait for the to dissolve - the water turns blue. Drop the keycaps in (and I swish them around for a minute).
Wait until the water turns clear (or brown, depending on how dirty the keycaps are).
Drain (I also rinse with cold water), leave to thoroughly dry.