What I posted is just somebody's guess as to what it would look like if it really exists. We do know that the Lockheed SR-71 spyplane has been officially retired and that no replacement has been named (as far as I know.) If there is a replacement, and it is a good bet that there is, it would be stealthy and superior in performance to the SR-71. So the artists conception is a reasonable bet as to what it would look like.
It could be what you saw. Maybe some of the aero-wiz types on FR could weigh in on the subject. I would like to hear their opinion.