Reciprocity failure, or maybe light loss from an extremely extended bellows in a close-up shot. You'd have to be shooting extreme magnifications for it to make your negative almost completely blank, however.
Reciprocity is the "rule" in photography that states doubling the length of your shutter will result in doubling the exposure the film receives (thus doubling the shutter gives the same amount of additional exposure as opening up your diaphragm one stop). Thus, reciprocity failure is when this fails to hold true.
It happens in exposures that are outside of the reciprocity range of the film. This range is usually different for each emulsion, and is listed in the data sheet.
It can be adjusted for, and certain films are much, much, much better than others at maintaining reciprocity during long exposures. In general, "old school" emulsions are among the worst offenders when it comes to reciprocity failure, while T-grained films tend to be the best.
The biggest problem, aside from the fact that you have to sit there all damned night with very long indicated exposures, is that once the initial testing for necessary adjustment is performed, the failure affects the emulsion differently with different levels of light. The areas of the emulsion capturing the dark parts of the composition suffer reciprocity failure more severely than the areas capturing the lighter areas. Thus, making an exposure when reciprocity failure is occurring also raises contrast. Again, worse with some films, better with others. It it usually recommended by manufacturers that you underdevelop your film to compensate for the high contrast that results from longer exposures.
