No, just a secret block that isn’t broadcast to everyone. 1 block a week is plenty to pull this off and that’s just 1/1,000th of the worlds mining power. Unless you get really unlucky and the block fails to enter the block chain letting someone else gets credit for the transaction.
I'm curious what it's like, so here is a small calculation. Some metrics:
- 144 blocks per day are mined on average
- the current network hashrate is 145M TH/s
- a 100 TH/s rig is about $10k.
The investment to be able to have full control of mining one block on average, without electricity, internet and storage :
- per week: you'd need 143k TH/s (145M / (144 * 7)), so about $14M of investment in just the mining rig (provided you can buy it all)
- per month: you'd need 33k TH/s, so $3M of mining rig investment.
- per year: you'd need 2.7k TH/s, so about $270k in mining rig equipment.
Of course, there are a lot of variables here (e.g hashrate is highly variable), but this gives a general idea.
All this for a "washing" method that heavily implicates the miner: the address of the new coins is still known, it's not really "clean", just an unusual transaction.
If you're really patient (and many big time criminals are), you only need to mine once every few months, maybe once a year or less. That's still hundreds of thousands of dollars of mining equipment, but well in reach for many.