2004 Volkswagen Passat EGR VALVE LEAKING SOMETIMES BLACK SM

Engine Mechanical problem
2004 Volkswagen Passat Engine Size unknown Two Wheel Drive Manual 88000 miles

HI, I have a 1.9 TDI Passat 130BHP. Sometimes I see pitch black smoke when I accelarate quickly in my rear view mirror. Also I have noticed my mpg is down quite a bit. I haven't realised any loss of performance. However, there is a large amount of oil leaking (black) from the EGR valve. One garage siad I should change it and another says the turbo may be going and this leak is normal. What do I do and can you shed some light on the smoke, lower mpg and leak? Should I change the EGR valve?