As far as I am concerned, 'Darkside' is the name of a northshore trail in Scotland.
I've not got a clue where it all started here with people using it to mean 'Road Cycling', but IMHO it is just daft.
Please just say 'Road Cycling' or even just 'Road'.
Or even better, go to a road cycling forum! Why does it need to be discussed in a MTB forum? How many threads on the popular road forum have 'muddyside' posts?