If the stem has softened, and there is a small amount of sawdust-like frass by the split, you have squash vine borer. These pests are the larvae of a moth, and feed on the inside of the zucchini stem by the base of the plant. To control, slit the stem open carefully along the main stem until you locate the worm (it is usually not far from the slit it made). Extract and destroy the worm.
If that isn't the case, then it is most likely caused by too much water. Squash plants crack sometimes, right after a heavy rain. This can't really be avoided, but watch for disease in the slit until it callouses.
Another way that a bush-type squash (because of the thicker stems) can crack, is rotating the stem to point another direction. This stresses the stem base, and it may crack. To avoid this, try not to move the stem.
From the picture, this looks like tension splitting, where the stem has a high enough tension in itself to split open. It shouldn't affect growth/production. This isn't a big deal concerning plant health, and you can mound soil around the stem if you want. Do not try to fix with wire, because this will not really help, but may damage the stem itself. Also, this will only happen when the stem 'ripens', or hardens, which sounds like what your plant has done. The stem hardens from the base out, and becomes darker in color - like you've described.