If the Sun & Moon should doubt They'd immediately Go out.
As far as we know, the sun and the moon aren't sitting around thinking about things and having debates about logic. They just radiate light (or, the moon reflects light, if you want to get technical).
They don't question the basic existence of some kind of reality—they are. By saying that if the sun and moon started to doubt, they'd immediately go out, Blake is saying that people who get wrapped up in doubting everything lose their ability to really exist. They're not doing the equivalent of what the moon and sun typically do, which is shine: they're not painting pictures or practicing medicine or putting out fires or discovering new planets or tending a garden or picking up garbage or catching fish. They're just doubting—and that's a total waste of time. It puts out whatever light we have access to.
To be in a Passion you Good may do, But no Good if a Passion is in you.
This is one of the couplets that doesn't really have a greater metaphor—it's more proverbial, just a little wisdom to lighten up your day.
Blake is saying that it's good to be passionate about something—he, for example, is obviously really passionate about poetry and art. There's nothing wrong with bringing a ton of energy and zeal to whatever it is you're into, but you won't be able to do any good if you're possessed by that desire or energy. If it's in control of you, and you're not in control of it, only some sort of disaster will result (or, you know, you could just end up irritating all the people around you).