General Synonyms:
* Stain
* Smudge
* Mark
* Spot
* Fleck
* Dab
* Smear
* Blur
More Specific Synonyms:
* Absorb: If you are talking about blotting up liquid.
* Deface: If you are talking about ruining something's appearance.
* Obliterate: If you are talking about completely removing something.
* Diminish: If you are talking about making something less significant.
Examples:
* Stain: The ink blotted onto the page. (The ink stained the page.)
* Absorb: She blotted her tears with a tissue. (She absorbed her tears with a tissue.)
* Deface: The vandal blotted the painting with spray paint. (The vandal defaced the painting with spray paint.)
* Obliterate: The fire blotted out the entire building. (The fire obliterated the entire building.)
The best synonym for "blot" will depend on the specific context of your sentence.