I realize that the spectrum is man-made, and "visible" is a human-relative term. However, the scale is based on objectives, such as frequency and wavelength, and "visible" light is objectively unique in that it wholly absorbed by solid/opaque objects. EM waves on either side of it can pass through intervening media (albeit to different degrees). Is there a specific reason why the "colored" wavelengths are uniquely blocked by objects?
I feel like Captain Obvious, but you seem to have it backwards.
Different objects (continuous media, conducting grids, whatnot) have transmission and absorbtion spectra, that is, they transmit various parts of the electromagnetic spectrum to different degrees. Our eyes, on the other hand, detect only a part of the radiation -- that part is what we call visible light, and the objects that block that light are (commonly) called opaque. Of course, opaque objects (for visible light) may be transparent to other frequencies (e.g. radio waves through walls), and transparent objects may be opaque to other radiation (e.g. UV through glass).
This is different from being solid: glass and bricks are solid, water and black ink are liquid, yet one of each transmits visible light and one doesn't.