I would argue that Blue Badge holders should not be exempt from LTN restrictions. Blue Badges exist to allow disabled people to park as close to their intended destination as possible, minimising the need for them to walk or wheel where this may be difficult.
I cannot see any justification for a Blue Badge holder to be exempt from a LTN, when they are driving (or being driven) they are simply traffic, the same as anyone else.
Also - there are 235,000 Blue Badges in use across Scotland. That would be a lot of exempted vehicles.