PayPal has obtained appropriate licenses to act as a money transmitter in most states [1].
I can't tell from a casual reading of Blockchain's site if they are doing anything that would require state licensing or not. It seems that they hold a backup copy of their user's Bitcoin wallet. If that is just a backup for the "real" wallet on the mobile devices, or just used for syncing copies of the wallet between the user's multiple devices, my current vague understanding of the rules is that this would not be money transmission. However, if their servers are acting as intermediaries between the user and third parties for the exchange of Bitcoin (for goods, services, or real or virtual currency), then I think that would make them a money transmitter (or even make them a bank, which also opens up a huge regulatory can of worms).
[1] https://www.paypal-media.com/licenses