All I said was:
"If Theory A is true, then we would see Observation B."
And you are turning it into:
"Observation B is evidence that Theory A is correct."
Which is faily reasonable in the context that a round earth would provide a visible horizon.
Not really. I didn't say anything about a round Earth providing a visible horizon at the exclusion of any other shapes.
However it's nothing like: "the sky would be blue on a round Earth, therefore the sky being blue is evidence for the Earth being round".
It's exactly like it. All I'm saying, in both cases, is that "if the Earth were round, we would see Observation B". You're the one changing it around to say "if we see Observation B, then that is evidence that the Earth is round".
You're failing to provide the link between the blue sky and the shape of the earth.
That's because there isn't one. You're failing to provide the link between a horizon and the shape of the Earth.