I couldn't find definitions for "blanket charge". What I've gathered from my Google searches is that the term seems to have two distinct meanings:
A lump sum that is charged (Google find: "…his services were not included in the $750 blanket charge.")
An overly generalized accusation (Google find: "The blanket charge is often made by TNOJ purists and others that Dixieland jazz was
merely the white musicians' watering down of the black players'
TNOJ.")
Is my assumption correct?
Best Answer
Blanket charge has these two different meanings because charge has two different meanings†: the payment required for goods or services and an accusation.
Blanket is a metaphorical use designating something which "covers" everything involved, as a blanket covers a bed. Blanket may be applied in this sense to other entities than charges: blanket statement, blanket insurance, blanket ban, blanket amnesty, and the like.
† Two different meanings which are relevant here—charge has other meanings as well.