He said that the sky [is/was] blue

[xterm]

He [believed/discovered/said...etc] that the sky [is/was] blue.

Is "is" or "was" correct here?
The sky is blue is a universal fact.

[Nightmare85]

***Neither a teacher nor a native speaker.***

I would say:
He said that the sky was blue.
He says that the sky is blue.

When the sky is not blue anymore, you can say:
He said that the sky used to be blue.
He says that the sky used to be blue.

Even if the sky is still blue, you should avoid mixing tenses like:
He said that the sky is blue.

However, in "real life" many speakers tend to break the rule in order not to confuse "normal" speakers.

Cheers!

[Rover_KE]

The sky is blue is a universal fact.

No, it's not.

This afternoon it was grey and now it's black with stars.

Rover

[freezeframe]

No, it's not.

This afternoon it was grey and now it's black with stars.

Rover

In the summer it's orange here... mmmm smog

[freezeframe]

This is not entirely related but this thread reminded me of this:

Distinguishing blue from green in language - Wikipedia, the free encyclopedia