[MyFairLacy]

Cherries are not harmful to dogs but the pit (or seed) is:

I found this on http://www.lacetoleather.com/fataltodogs.html

Apple seeds, cherry pits, peach pits, pear pips, plums pits, and apricot pits contain cyanide, which is poisonous. While a few apple seeds may not cause a problem, the effects can accumulate over time if they are given to dogs regularly. Dogs should not be allowed to chew on a peach pit, cherry pit, apricot pit, or plum pit. Chewing can allow ingestion of cyanide. Chewing could also result in the pit being swallowed, causing continuous exposure to cyanide, or could cause the dog to choke.

A news story about it http://www.thepittsburghchannel.com/...76/detail.html

I don't know if that causes dark urine or excessive thirst but I'd look into it. Your dog could also just have a bladder or kidney infection