CJ, just curious if you checked the rubber hoses that run in between the injectors. On my 240D, I could smell raw fuel as soon as my wife pulled in the driveway recently, but when I went to look for a leak the next day, I couldn't find one. So I temporarily sprayed a little cheapo paint on all of the fuel lines under the hood & drove the car around town for a while ... came back & I still couldn't find a leak. I drove it on the interstate ... came back & I had VERY small stream of fuel (& cheapo paint - which made the leak obvious) coming from the end of the hose on the #4 injector.
As far as the blue smoke, have you had a thermo recently? If your not burning any oil, then blue smoke is usually a sign of unburned /under-burned fuel ... which would also drop your MPG's. My guess would be the thermo since you don't have any other obvious symptoms.
1980 Mercedes 240D
1982 Mercedes 300SD
1985 Mercedes 300D
1993 Dodge Cummins Turbo Diesel