Bit of paint (4)
(Other definitions for blob that I've seen before include "Something small, soft and round" , "Globule; round spot" , "Indistinct shape" , "Indistinct object" , "Small piece signifying nothing" .)
'bit of paint' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for dab that I've seen before include "Flatfish; fingerprint" , "Bad kind of fish" , "Expert; flat fish" , "Press gently; flatfish" , "[SOMETHING FISHY]" .)