There is no way to detect a illegal transaction.I doubt even in future this feature will ever be implemented.How can you decide if a transaction is illegal ? Volume of the trade ? Bitcoin address associated with it ? All the blockchain does is process the transactions by keeping them public regardless of who or what is being the address is used for.
Yeah sometimes blockchain check on some mysterious transaction such as what you have said. And blockchain doesn't just process transactions .
What you are saying here is incorrect or an error in translation. The blockchain doesn't 'check on' mysterious transactions, it just lists all transactions, mysterious or otherwise, it's a public ledger.
People can use it to check for suspicious transactions or whatever but the blockain itself does nothing of this sort.