This was very much in the class of the Blinking Nebula. When I looked directly at it, it would completely disappear.
The Nebula appeared much smaller than I was expecting. The Bright Star Atlas (BSA) gives its size at 74"x42". Burnham's Celestial Handbook states it as 50"x20". I thought it was more like the size of I418, which was only 14"x11". Perhaps I was only seeing the inner core of the Nebula.
The Nebula also appeared fainter that I418, even though BSA lists them as Mag 9.3 and 9.4 respectively. Nevertheless, it was much brighter than Burnham's figure of 11.5.
I may have being using my imagination to see blue light from the nebula. It certainly was not like many planetaries where this is a notable feature.
Internet articles list the central star as a white dwarf with an extreme surface temperature of 200,000 Kelvin. Its distance is approximately 4000 light years.
I used my Tak 100D to make these observations.
An Astronomy photo of the day for NGC 2440:
https://apod.nasa.gov/apod/ap070215.html