There is no accepted means to measure blow-by and therefore there can be no hard and fast acceptance criteria for blow-by. In general all internal combustion engines exhibit blow-by as the sliding contact between the cylinder and the piston rings is not a zero leakage interface. Surface defects from wear typically increase blow-by as they interfere with the formation and durability of the oil film seal that forms in the designed in surface finishes of the interface. In a Diesel, higher than "new condition" blow by can typically be tolerated with the engine running mainly because the blow-by products can be routed into the intake and consumed with the Diesel fuel without adversely affecting the performance, until the blow-by is pretty extreme. The first sign of blow-by being a problem is usually cold starting, when the leakage effect is highest because the engine is turning the slowest (more time for pressure to leak past the defective seal) and the heat of compression is vital to achieving ignition. Once started, the engine speed usually makes the threshold for acceptable blow by increase. I have seen some pretty severe cases that, once running stink a bit, but run smoothly and with reasonable power.

So, a good guide might be that if you can start in cold weather, blow-by is likely not a concern for you. This assumes the battery and starter and oil and fuel system and glow plug system are all up to snuff as any one of them in less than good condition can lead to extra stress on the others, and a starting problem.

On my 1982 240D I have a enough blow-by to make the cap, once unscrewed, float off the valve cover and fall to the ground. No geyser of oil mist but not something that is hard to see either. This car starts right up in single digit Fahrenheit weather, so I would guess that amount of blow-by is not compromising performance. I get oily build ups around the cyclone separator connections and some forced oil leakage under the valve cover gasket between valve adjustments (typically 15,000 to 20,000 mile intervals). The engine can look pretty messy, but cleans up nice when I do service (I learned from others on this forum to wash the engine before I do work on it, like valve adjustments and recommend that for any DIY maintenance under the hood).

Hope this helps, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)