Why DARPA wants blockchain based secure messaging system DARPA (US Defense Advanced Research Projects Agency) has released a Request for Proposal for a Secure Messaging Platform using a blockchain framework. 

The following is the Objective paragraph from DARPA's call for proposal:


  OBJECTIVE: Create a secure messaging and transaction platform that separates the message creation, from the transfer (transport) and reception of the message using a decentralized messaging backbone to allow anyone anywhere the ability to send a secure message or conduct other transactions across multiple channels traceable in a decentralized ledger.


The request for proposal is available in full, here:
DARPA Call for Proposal for Secure Messaging Platform.


Why are contemporary secure messaging platforms like TextSecure, Cryptocat, XMPP/OTR, Bitmessaging and so many more NOT enough? 


What do these contemporary solutions lack, as the Secure Messaging Scorecard suggests many applications have all these features?

What does blockchain provide when it comes to Secure Messaging and why it is necessary?


How can Military Interdepartmental Purchase Requests (MIPR) be implmented using the secure ledger or blockchain?

In particular, this paragraph of the proposal is not making sense to me

The messaging platform would act as the transport for a cyptographically sound record of all transactions whether they be MIPRs, contracts, troop movements or intelligence. Troops on the ground in denied communications environments would have a way to securely communicate back to HQ and DoD back office executives could rest assured that their logistics system is efficient, timely and safe from hackers. The benefits are broad and could even be applied to domains such as space. With crowded skies it’s important to maintain situational awareness of all satellites and those concerned with space situational awareness/telemetry or air traffic control could instantly share data between nations using a separate but equivalent ledger implementation thus removing questions as to the authenticity and integrity of the data.

What does transport mean here?
How does blockchain help troops communicate in denied communication environment help?
I am also not clear about the argument on situational awareness of satellites and how does blockchain enable sharing data between nations.

Simple answers would really help me and others like me. Thanks
As I see it, the key part is at the very end: "traceable in a decentralized ledger".  That feature isn't described in the EFF page you cite, and I'm not aware that any of the services listed provide it.

To see why this is important, imagine a scenario like the following:

Major Smith receives a message at 0400 hours from Battalion HQ about impending enemy action.  But he's too busy playing Angry Birds and doesn't bother to pass it along to his units in the field.  The enemy attacks at 1100 hours, the field units are unprepared, and they are defeated with heavy casualties.  Major Smith is afraid he will be court-martialed for failing to share the intelligence with his troops.  So he writes a message, backdated to 0500 hours, addressed to Captain Jones and ordering him to contact the field units and give them the information.  He also secretly orders (or bribes) Corporal Johnson, an IT administrator, to alter the message system's logs to show his forged message as having been sent at the appropriate time.  This way, he can blame Captain Jones for failing to act on the message, or at a minimum, plausibly claim there must have been some sort of communication failure that kept Jones from receiving it.

If there is a secure decentralized ledger of all messages sent, then Smith can't insert his fake backdated message in it, and there's no single person he can bribe in order to get it done.  So he can't dodge responsibility for shirking his duty.

A secure decentralized ledger is precisely what a blockchain provides.