There are a couple possibilities not mentioned so far.
You didn't say if you took your foot off the gas when you shifted,
but if you did, this makes a pretty high momentary manifold vacuum
and if your intake valve guides are worn much, this will dump some
oil into the intakes and cause blue smoke when you back off.

Another possibility is that some auto trannies have a vacuum operated
valve that gets its vacuum from the intake manifold. If you have one
of these and it is starting to go bad (leaky diaphragm or some such)
it can pull tranny fluid into your intake manifold.

The quick and dirty cure is not to look in the mirror after shifting....
--
George Downs, Bartlesville, Oklahoma, Central US