Is there an "SR-72" under development?
Yes, the SR-72 project has indeed been officially announced. Lockheed confirmed it to Aviation Week back in 2013. Apparently it actually did get quite a bit of attention at the time, enough to actually cause Aviation Week's servers to go down for a while after the announcement.

The source quoted in the question is correct that Lockheed's announcement did confirm that the SR-72 would be hypersonic:


  But now Lockheed Martin believes it has the answer. “The Skunk Works has been working with Aerojet Rocketdyne for the past seven years to develop a method to integrate an off-the-shelf turbine with a scramjet to power the aircraft from standstill to Mach 6 plus,” says Brad Leland, portfolio manager for air-breathing hypersonic technologies.


Also, from Lockheed Martin's own website, confirming both the Mach 6 capability and the intention for it having at least the option of flying unmanned:


  Envisioned as an unmanned aircraft, the SR-72 would fly at speeds up to Mach 6, or six times the speed of sound.


According to Lockheed, a technology demonstrator aircraft may be flying by 2018:


  Lockheed has run scaled tests on components. “The next step would be to put it through a series of tests or critical demonstrations,” Leland says. “We are ready for those critical demonstrations, and we could be ready to do such a demonstration aircraft in 2018. That would be the beginning of building and running complete critical demonstrations. As of now, there are no technologies to be invented. We are ready to proceed—the only thing holding us back is the perception that [hypersonics] is always expensive, large and exotic.”


Here's a rendering of the proposed design from the SR-72 Wikipedia page:

Source: Wikipedia, which apparently took it from Lockheed's website.

Lockheed was recently awarded a grant from NASA to study the viability of their proposed combination turbine/ramjet engine solution for the SR-72.


  Task Order No.NNC15TA03T provides for a parametric design study to establish the viability of a Turbine Based Combined Cycle (TBCC) Propulsion system consisting of integrating several combinations of near-term turbine engine solutions and a very low Mach ignition Dual Mode RamJet (DMRJ) in the SR-72 vehicle concept. Task Order NNC15TA03T is issued under Contract NNC10BA08B on a firm fixed price basis. Firm fixed price is $892,292.