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We model the collapse of a spherically symmetric, constant density,pressureless shell onto a preexisting black hole. Contrary to a recent claim ofLiu and Zhang (2009), we show that an observer at infinity never sees any partof the shell cross the event horizon. The entire shell appears to "freeze"outside the black hole. We find that during intermediate stages of thecollapse, the change in apparent size of the inner surface is non-monotonic.However, at late times, the apparent sizes of the inner and outer surfaces ofthe shell both approach 3\sqrt{3}GM/c^2 asymptotically, where M is the combinedmass of the initial black hole and the shell.

Apparent motion of a spherical shell collapsing onto a black hole