Surely not a great tailed grackle?

Firstly we have to rule out the most common answer and this would be Magpie.

You say your dad said the feathers were all black - from this angle and looking towards the sky would usually result in the bird appearing darker.

You say that your dad said the body was like a Magpie but thinner - so 'like a Magpie' sort of indicates that it was a Magpie - and your dad telling whether a bird is 'thinner' than what they should be is fairly impossible as individuals vary as well as other factors come into play such as simply the posture a bird adopts to stay balanced against the wind especially on a wire.

It would be very very very unusual to be a Great Tailed Grackle - not that that isn't impossible I guess in the bird-spotting world.

But all evidence points at a common Magpie for me.