It is true that in the past, Bitcoin and other crypto were used in the Drug Trade because some people thought it was "safer" in terms of anonymity. However times are changing, and Bitcoin's use in those illegal activities decrease drastically. Let me drive you into this !
First let's bring back the past, in 2008 Bitcoin was created as an electronic peer to peer payment system and yes Bitcoin was used on the Dark Web to buy guns, drugs and other illegal things. Indeed people have more anonymity there than on the regular Web, and Bitcoin appeared to be a suitable currency for those markets. Some studies estimate the amount of Bitcoin use for illegal purchases to $707 million in 2017.
However times are different now, indeed it turns out that the percentage of Bitcoin transactions involved in criminal activity fell from 30% in 2012 to 1% in 2017. Also using BTC to buy drugs leaves an evidence that law enforcement agencies manage to find more easily. Bitcoin transactions occur on a decentralized, public blockchain, making all activity traceable (thanks to KYC), so the US Justice is more and more efficient on those cases and in 2017 manage to seize more than 150 000 Bitcoin from those illicit transactions. Furthermore Bitcoin is gradually becoming a mainstream method of payment and to invest.
We can compare it to cash, the level of criminality involved with cryptocurrencies, with Bitcoin as the leader, is way smaller. In cash, we are talking somewhere around $2 billion to cloak criminal activity every year. In contrast the cryptocurrency market is somewhere around $180 million.
Without a doubt Bitcoin is still used for those kind of transactions. But it is now more and more difficult for people to use it for illicit activities and to remain anonymous. Moreover Bitcoin is becoming a more attractive currency to legacy institutions like hedge funds and pension funds as an investment vehicle, otherwise its use on the Dark Web won't make it viable.
To sum up, in the past Bitcoin has been used for illegal purposes and fortunately today it is less and less involved in those activities thanks to KYC and some other rules. It is better for Bitcoin to be a clean currency for investment purposes, money transactions and more, in this case Bitcoin will continue to move in the right direction.