It`s hard to say how many transactions on BTC are used for DNM-purchase but i think it`s round about 2-3%.
If 50% of there 2-3% move over to XMR, the price should double very quick.
The forfeiture complaint issued by the justice dept re AlphaBay detailed almost 840,000 BTC passing through the AB's accounts between May 2015 and Feb 2017. As someone else pointed out, one might double that to account for the next two largest marketplaces in the DNM.