As a Blue BAdge holder I can assure you that not all spaces are free.
Within 10 miles of home I am in 3 different boroughs which offer 3 different schemes ranging from completely free, first 3 hours free then pay and pay full whack for parking in marked bays. Unmarked bays are different too!
And also other parks (i.e. NCP) have bays but (example multi storey car-park) where you get a timed ticket to go in, go to machine to pay before leaving and you have to pay full whack as there is no reduction system available to get your ticket stamped to let you out.
Even hospitals have different systems - generally meaning you pay or at best 3 hours free - but what if you are there all day for tests??
HAving said all this I do appreciate the sytem and hope it will continue