DIAGNOSIS The container is almost certainly broken. The blade assembly is permanently attached to the container with a seal. If liquid is coming out from between the black blade base and the clear plastic, that internal seal has failed or the plastic has cracked. This is not a user-repairable part.
FIX You need a new container. Unplug the base. Unscrew the old container from the base (turn it counter-clockwise). Order a genuine Vitamix 64-ounce "Low-Profile" container replacement (the standard one for the 5200). When the new one arrives, screw it onto the base clockwise until it's hand-tight. Do not overtighten it with tools.
IF THAT DIDN'T WORK Before you order, double-check that the leak isn't actually coming from the lid or the center plug of the lid. Fill the container with just water, seal it, and turn it upside down over the sink. If it drips from the top, the issue is the lid seal, not the container base, and that's a much cheaper part to replace.
HEADS UP This is a common failure point over many years of use, especially if the container has been dropped or hit against a faucet. It's a wear item. Avoid twisting the container on and off the base with excessive force, as that can stress the connection point.