Edit: Also, the use of "anonymizers" in the bitcoin ecosystem is common. Silk Road has something they call tumbler which masks transactions in such a way that you can't really say for sure where the bitcoins are coming from.
https://en.bitcoin.it/wiki/Anonymity
I assume that "tumbler" is basically as good as any eWallet service with a bit of obfuscation on the transactions in and out. That is fine, if you trust someone enough to take you coins, mix them with a bunch on other peoples money, and then send them back to when you need them, you can remain pretty anonymous. This is because that person/organization can do virtual funds transfers off of the bitcoin block chain record. However, in the scenario where address are marked as dirty or clean, that eWallet supplier is going to find him/herself in the possession of many dirty bitcoins and will have to burn them.
In the Silk Road example, if people use stolen bitcoins on that network, all bitcoins that get sent in are tainted by that (to some percentage) and the value of the entire working volume of Silk Road bitcoins in the "tumbler" laundering machine is lessened. This means that either Silk Road passes that loss of value on to the sellers, possibly pissing them off, or they throw those bitcoins away (or send them back) and tell the person paying with them to pay again from a clean address because that money was reported as stolen.