Something indistinct and black that may be on book's cover (5)
I believe the answer is:
blurb
'that may be on book's cover' is the definition.
(a blurb appears on the back cover of a book)
'something indistinct and black' is the wordplay.
'something indistinct' becomes 'blur' (something blurry or indistinct).
'and' means one lot of letters go next to another.
'black' becomes 'b' (as in pencil classifications).
'blur'+'b'='BLURB'
(Other definitions for blurb that I've seen before include "Promotional text for a book" , "Short description on a book, etc" , "Book endorsement" , "Marketing text" , "Commendations on book jacket" .)