First of all, I have no idea what the black in the mortar could be.
At a guess I would say it could be an iron compound but that would rust, or black powder used to colour the mortar, or indeed black, rotting pieces of wood/charcoal. It will need to be analysed.
You have a Bellway house and Bellway have some history in building homes with weak mortar,
this instance had disastrous consequences for the home owner. In this instance the roof repair was "botched" and the mortar was found to be weak and the wrong specification of bricks were used. I believe the NHBC bought the house and it was demolished although "Col" probably signed a Confidentiality Agreement aka "gagging order".
It is possible that frost is the cause, with the action of freeze-thawing.
It could also be that the mortar had not sufficiently cured (and/or was not protected) and was subject to freezing temperatures.
The mortar [it looks like] may not have sufficient cement in the mix but this would need to be tested in a laboratory to confirm.
Is this in just one section or band (known as a lift in the industry) of about 1200 to 1500mm in height or is it on a whole elevation?
If it was a band it could be just frost damage.
You could use a car key and twist in into a joint and see how far the mortar crumbles and it goes in.
If it stops or is harder after 12mm or so then re-pointing may be a sufficient remedy.
What you really must do is WRITE to the NHBC now, notifying them of this. It is structural and they may have to sort this out in the years to come!