Huddersfield - Walkden is associated with
Bolton, Salford Crescent and Wigan Group
The single fares for Huddersfield - Walkden are:
SDS Any Permitted £11.80
SDS Via Hebden Bridge £11.60
Huddersfield - Bolton is £15 SDS, and so Bolton is not a valid routeing point
Huddersfield - Salford Crescent fares are the same as Walkden, so Salford Crescent is a valid routeing point.
Huddersfield - Wigan Stations single fares are £27.50, £25.40 and £17.10, so Wigan is not a valid routeing point.
Huddersfield - Salford Crescent is mapped:
MR
This permits
Huddersfield - Stalybridge - Manchester (Piccadilly or Victoria) - Salford Crescent - Walkden (these are both also permitted under the shortest route rule)
only
However the ticket is routed 'via Hebden Bridge'. So you must travel via Hebden Bridge, and as this is not on a permitted route, the permitted route is:
Huddersfield - Hebden Bridge
Hebden Bridge - Walkden
The route Huddersfield - Hebden Bridge - Rochdale - Manchester Victoria - Walkden is the shortest route and is permitted.
Huddersfield - Hebden Bridge SDS is £5.60 Any Permitted
Routeing points are:
Accrington
Halifax Group
Manchester Group
Huddersfield - Accrington is £20+ - fails fares check
Huddersfield - Halifax is £3.80 - passes fares check
Huddersfield - Manchester is £11.80 - fails fares check
So only Halifax Group passes.
Hence:
Huddersfield - Hebden Bridge direct, or via Halifax, are both permitted
From Hebden Bridge to Walkden
£10.20 SDS Any Permitted
Manchester - Walkden obviously passes
Halifax Group - fails
Accrington - passes
and then:
Hebden Bridge - Bolton - fails
Hebden Bridge - Salford Crescent - passes
Hebden Bridge - Wigan Stations - fails
So the mapped routes are:
Manchester - Salford Crescent (direct only route)
Accrington - Salford Crescent - Accrington - Blackburn - Bolton - Salford Crescent is the only permitted route
So the full list of permitted routes for this ticket are:
Huddersfield - Halifax/Sowerby Bridge - Hebden Bridge
then
Hebden Bridge - Manchester Victoria - Walkden
or
Hebden Bridge - Blackburn - Bolton - Salford Crescent - Walkden
So yes, it is valid via Blackburn. Confirmed by NRE:
http://ojp.nationalrail.co.uk/service/timesandfares/HUD/WKD/211013/1515/dep?via=Darwen