I was trying out Kodak Portra 400 color negative 120 film in my Hasselblad 503cw last month.
Just got the negs back from the lab and they appear to be black and white.
Can someone explain how this is possible?
They may have been "souped" in the wrong chemistry. For all the differences between colour and black-and-white films, they are, at heart, the same thing. The colour films simply have extra bits of chemistry, both in the film itself and in the developing chemistry, that adds colour to what is really just a black-and-white image.
In black-and-white photography, you simply cause metallic silver to precipitate out of silver halides that have been made reactive by exposure to light, then get rid of the remaining silver halides in the fixing process. In colour films, there are dye precursors that attach to the silver, and in the "blixing" process, the remaining halides and unattached dye precursors are removed ("fixing" the image) and the metallic silver is removed as well ("bleaching" the image), leaving only the dye clouds.
If the silver is not removed (that is, if there's something wrong with the bleach/fix bath, either due to depletion or an incorrect mixture) you will get a sort of colourized black-and-white image; if the developer chemistry does not convert the dye precursors to dye clouds (that is, if the chemistry used is a black-and-white developer rather than the usual C-41 chemistry), then you'll get essentially the same result as with with a black-and-white film (but with a colour cast to the negative).
If it were the film at fault (let's say, for argument's sake, that the part of the dye that lives in the emulsion went missing during manufacture), then the blixing process would have removed the silver, leaving no dye clouds behind, and your negs would have been either blank or just the faintest ghostly traces of density.