***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.
- For Teachers