The quality or state of being blotchy; the appearance of having irregular spots or discolored patches.
From blotchy plus the noun-forming suffix -ness, which turns adjectives into abstract nouns describing their essential qualities.
English loves the -ness suffix for turning feelings and qualities into things we can count—blotchiness is the abstract idea of 'blotchness,' so you could say 'the blotchiness of her skin' or 'the blotchiness of the painting.'
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