Will getting cheek implants also fix my undereye darkening?

No, this is unlikely to improve your under eye darkening. A combination approach will give the most improvement. Fillers, lasers, and microneedling/PRP can improve under the eyes. My patients try to get non surgical treatments to avoid surgery. We use PRP and stem cells there often. I suggest consulting with an expert to develop a treatment plan best suited for your short and long term goals. Best, Dr. Emer

No, this is unlikely to help your under eye hollows. Under eye circles can be permanently corrected with surgery or temporarily camouflaged with fillers (Restylane, Belotero, Volbella).There are great non-surgical options to fill in under eye depressions and hollowness. Using dermal fillers, a liquid eye-lift can be performed. A dermal filler such as Restylane would be placed in the cheek and tear trough to hide the depression or bag underneath your eye. 
Surgery would require a fat-repositioning lower lid blepharoplasty. In this surgical procedure, fat from the eye area is moved into the tear trough depression filling the actual depression. 
I hope this helps!