Color temp of the ambient light would certainly be an issue for metering for black and white. After first reading about this effect in a magazine article, I performed a test myself. I spot metered the black cover of a stereo speaker (black with some detail) in daylight and tungsten, using a zone VI pentax meter. I bracketed shots on the same roll of film and printed all negs for same enlarger time to produce same black tone on paper. the neg metered in tungsten light required just over a stop of additional exposure to produce the same detail in the black as the daylight metered shot.

Since then, I've tried to add a stop of extra exposure whenever metering in tungsten.

I never tried this with different meters, but now I suppose I ought compare the Zone VI meter with the ttl meter in my 35mm camera...

What's really disturbing is that my Zone VI meter is Japanese, which according to Gossen is already calibrated for 3200 k. Does this mean that the gossen, calibrated for 5600 k, is off by 2 stops in tungsten light?