Is there any plan to promote XMR to darknet.
Forcing Darknet adoption, requires implementation of Multi-Sig-Escrow into Monero.
(Major) Dark Markets won't introduce/accept any coin without Multi-Sig-Escrow, as they all use MS-Escrow solutions for customer fund protection, against hacking, scamming, busts by LE, ...
It's not widely discussed on forums (for obv reasons) or advertised as important project goal, but I think its consensus in the XMR community, that XMR will be "Dark Market"-compatible (safe, easy handling/escrow/payment, trusted, ...).
So XMR will participate in the upcoming battle of the existing anon coins, to become the leading, dominant Dark Market coin.
To answer your question, when Monero (with all the needed features & ecosystem around) is ready and tested, in maybe 12 months. There will be an solid attempt to get XMR accepted and implemented at the leading major 3 till 4 Dark Markets (including informing DarkMarket community about XMR, ...), in the hope of becoming the leading DarkNet trading coin.