what color should my 71 340 motor be?

What is the approximate date of the color change to blue?

Is it just color or was there a Hp change as well?

I think March/April was the color change.

There is no known exact color change date but it was sometime from March to April of 1971. There was no HP change but some engine parts changed at either the same time or close to the same time. Passenger side valve cover went to a small 318 style center wire harness hold down from the previous wide version. Intake casting was the same number but a different casting which had new physical features, some (or all) negative battery cables were getting painted with engine color (so far all I've seen are previously bare in 71). Late (LATE) production cars could have electronic ignition. There were likely other small details that have yet to be recognized, it's a puzzle that is still not completely solved.

Just a FYI there is 2 different color orange paints for Mopars. Your 340 will need "Street Hemi Orange" to be correct, not "Race Hemi Orange". Also the small hose between the intake and water pump should be installed before painting and recieve over spray to achieve the factory look.