This statement expresses roughly the same amount of certainty as An helíyel swa-n kílu ("the sky is blue"), but also says that the speaker got the information from direct perception (especially sight). Contrast with the version without hwel, where there's more uncertainty about whether the sky actually is blue.
This statement expresses roughly the same amount of certainty as An myau swa-n swatel ("the cat looks black"), but also says that the speaker got the information from direct perception (especially sight). The verb also changes back to the indicative mood.
An myau swa-n swatel lú (with neither the subjunctive nor hwel) is ambiguous between this meaning and the previous meaning, but is allowed in informal speech.
Defocusing lú implies that the cat being black is new, important information, and the fact that I'm happy about it is not the main point of the sentence.
"Hope you're happy hearing Conlangery". Perhaps I should record this and send it in to Conlangery.