[DQDick]

Jcurtis has pretty much covered it, but there is one more thing it could be. If your black valve ends the pex run then you have a vent that opens in place of having the plumbing vented to the outside. Keystone usually uses these on kitchen sinks, but who knows. Anyway if it's one of those look for cuts, scrapes or a loose connection. The line would only leak if the valve failed to open and water backed up to it.