The burn is flowing towards the photographer's position. The picture was taken from the little footbridge shown in
NS5162 : Footbridge over the Blacksey Burn.
On the map, the boundary that corresponds to the line of the burn is marked "Burgh Const., P Const., PER & UA Bdy".
The present picture shows the artificial channel through which the burn flows, explaining its unnaturally straight course, as depicted on the OS map. From the bridge, the burn flows only a few metres before joining the White Cart Water, as shown in the following picture, a view in the opposite direction from the same point:
NS5162 : The Blacksey Burn.