How will you do to implement the notification feature, if you are a developer of blockchain.info?
Is receiving notification possible without ext_private_key?
No extended private keys.
Notification feature is challenging unless you want to trust someone with read-only access to your funds.
You effectively encrypt with two keys, a read key and a spend key, both of which can be derived from your public key.
The sending client would have to facilitate the observer.
Sending client and receiving client are decoupled, which means that the sending client do not know whether or not the receive side need such read key(e.g. for notification), when generating the transfer transaction.
My understanding is that there are only two options, in the protocol or not. If it is not in the protocol, then it will be very difficult to support such feature in the network.
By the way, are the read key or spend key the fields user provided in the name account for the sending client to use? So you mean the receiver will tell the sender whether or not facilitate the read observer according to account settings?