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Yes, black/cblack reflects

Yes, black/cblack reflects different black level specifications in RAW metadata
black - single black level
cblack[0-3] per channel BL in channel numbering order (channel numbers are returned by COLOR(..))
cblack[4-5] defines pattern BL (e.g. 6x6 for X-Trans)

As a rule: black is base level and cblack is correction, so if black is non-zero and cblack[.. is non-zero (for given channel or row/col), resulting BL for pixel/channel is a sum of black + cblack[channel] + cblack[pattern]. In practice all three are rarely present.

white is either format maximum (defined by bits per pixel), or maximum value defined by metadata (DNG whitelevel).
linear_max is 'specular white' read from metadata (if present in file)

-- Alex Tutubalin