We discuss how the Mg I λ285.21 nm line is formed in the context of standard plane-parallel modeling. The line appears to be very sensitive to the nonlocal radiation field determining the balance between neutral and singly ionized magnesium. We resolve between conflicting results in earlier λ285.21 nm line modeling by showing that, in the quite Sun, the line forms at sufficiently low density for partial frequency redistribution to take effect and give rise to small emission reversals in the core. We find this to be true only if we take proper account of UV line blanketing at the relevant Mg I ionization edges. In this case there is good agreement between theoretical line profiles and spatially averaged spectra from the Solar Maximum Mission (SMM) Ultraviolet Spectrometer and Polarimeter (UVSP) and from the French RASOLBA balloon experiment. Spatially resolved spectra obtained with the latter instrument show considerable variation in the line core, with emission present only in some locations and absent in others.