Blocking the center of the rear element usually reduces transmitted light first, and the visibility of the obstruction depends on lens design, focal length, and aperture.

From the community examples: a central obstruction on the rear of a telephoto tends to show up more than on a wide-angle lens, and it becomes more noticeable at smaller apertures than at wider apertures. In many practical cases, small central obstructions mainly just cut light with little obvious image degradation.

A simple estimate for light loss is by area: if 10% of the effective optical area is blocked, roughly 10% less light gets through. That does not usually remove part of the frame outright; instead it may cause dimming or, if large enough, visible image artifacts.

If the blocked portion is large, image quality can change. Historically, large central obstructions have been used to create soft-focus effects by forcing more of the image to be formed by peripheral rays, which are often less well corrected.

So: small blockage = mostly light loss; larger blockage = possible visible darkening and optical artifacts; telephotos and stopped-down apertures tend to reveal it more clearly.