Having the most blotches or discolored patches on a surface.
From blotch (a dark spot or stain, possibly from Dutch blot) plus the superlative suffix -iest, making it the most extreme form of blotchy.
Blotchiest is rarely used because we usually just say 'most blotchy' instead—but it's technically correct and shows how English lets us stack suffixes to compare qualities, even when it sounds awkward.
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