I recall getting an Oily Blackmouth from the pool outside the entrance to Stratholme. That leads me to believe that once you've left your race's newbie zone, if you fish from any lake or coastline in Azeroth, you will have the possibility of getting an Oily Blackmouth.
If you're fishing in Wetlands or Westfall, they're among the best fish you can catch. If you're fishing in Feralas or AzerothAzshara, they're among the "worst" you can get. That's not bad.
A "good place to fish for them" will depend on your level and fishing skill.
If my goal were to only catch Oily Blackmouths, I'd look for schools of Oily Blackmouth along the coast of STV or along the Tanaris shoreline.
Edit- Proofread is my friend :-( TYVM ohmikeghod for the obvious.
Edited, Dec 7th 2007 2:42pm by HoyadinFTW