Many locomotives operated in "predecsessor paint" into the mid 1980's..
I started taking train photos in 1983, and I saw several units in PC black into 1985..but none after that.
I have read that people saw a few operating into the late 80's..
but by 1990 it was definately over, and everything was in CR blue through the 90's..
(yes, several locos remained in predecessor paint after that, but they were stored or sold locos..not actually operating with Conrail)
So I would place the date for "All Blue" around 1987 or 1988..
As for the oldest Conrail locomotive still operating today, I would be willing to bet its an EMD SW1 from 1939 or 1940..
those were about the oldest locos that made it to the CR roster from the predecessor roads..
I have a few listed on my NYC, LV and EL survivors webpages:
Im not sure which is the oldest, or which is the (oldest + still operating),
but we could probably figure it out with a little research..
I doubt you find much of anything older than a 1939 or 1940 SW1 still operating today..
(probably the *oldest* surviving loco that actually operated with Conrail is "Old Rivits"..the first GG1..she was built in 1934!
but obviously she is no longer operating today..)