I can't think of a single good reason why harmonised black should be used as the neutral. There's logic and even guidelines to use the grey, but nothing suggesting black.
Here are some examples of how the harmonised colours can get used for different applications.
3 core and earth for L, sw L, N
brown L
black sleeved brown sw L
grey sleeved blue N
SWA 3 core for L, N, E
brown L
black sleeved green and yellow E
grey sleeved blue N
SWA 3 core for 3 phase
brown L1
black L2
grey L3
armour E
SWA 4 core for 3 phase
brown L1
black L2
grey L3
blue sleeved green and yellow+armour E
SWA 4 core for 3 phase+neutral
brown L1
black L2
grey L3
blue N
armour E
Don't think any of these are interchangable, other than the 3 core+E at the top of the list.
Possibly people do the SWA 3 core for L, N and E differently, but again, grey as N for me.
Here are some examples of how the harmonised colours can get used for different applications.
3 core and earth for L, sw L, N
brown L
black sleeved brown sw L
grey sleeved blue N
SWA 3 core for L, N, E
brown L
black sleeved green and yellow E
grey sleeved blue N
SWA 3 core for 3 phase
brown L1
black L2
grey L3
armour E
SWA 4 core for 3 phase
brown L1
black L2
grey L3
blue sleeved green and yellow+armour E
SWA 4 core for 3 phase+neutral
brown L1
black L2
grey L3
blue N
armour E
Don't think any of these are interchangable, other than the 3 core+E at the top of the list.
Possibly people do the SWA 3 core for L, N and E differently, but again, grey as N for me.