Monero (XMR) is the obvious choice for darknet markets, since Monero is completely anonymous. Literally, with Monero the destination, origin, and amount of a transaction is completely obfuscated and cryptographically encrypted. In fact, Monero is pretty much the only cryptocurrency that blockchain forensics hasn’t cracked yet, and due to this Monero is not available on most exchanges, since exchanges cannot do anything to ensure that Monero isn’t used for illegal purposes. 

On the other hand, Bitcoin (BTC) is relatively non-anonymous, with the amount, origin, and destination of each transaction being published on a public blockchain. Blockchain forensics technology has made it straightforward to connect Bitcoin addresses to identities, and from there law enforcement can take action against anyone conducting illegal activities with Bitcoin.

Despite the obvious superiority of Monero over Bitcoin for anonymity, apparently a survey of 49 darknet markets found that 98% of them accept Bitcoin, while only 45% accept Monero. 

Essentially, only 22 of the 49 darknet markets in the survey use Monero, with 26 darknet markets not accepting Monero and instead using Bitcoin.

This seems really nonsensical, since using Bitcoin can easily lead to the users of a darknet market being arrested. It is especially nonsensical since the risk can largely be avoided by using Monero.

Perhaps the reason why Bitcoin remains dominant over Monero on the darknet is that Bitcoin is much easier to obtain. Basically, most exchanges do not offer Monero, and therefore most people do not have Monero, whereas Bitcoin is available on every crypto exchange. 

Therefore, darknet markets need to accept Bitcoin in order to be easily usable, even though it is extremely risky. 

If this is the case, then the Monero blockade, i.e. most exchanges not offering Monero, is working as intended. If darknet users are being forced to use Bitcoin instead of Monero due to a lack of Monero availability, it makes it much easier for law enforcement to crack down on both darknet market users and vendors.