Did Blake ever provide his own commentary on this piece?
Kind of! Between 1805-1810, Blake created over a hundred paintings illustrating scenes from the Bible. Among these was a four-painting cycle from the Book of Revelations, or The Apocalypse, in which John the Evangelist describes his vision of the end of the world.
The closest thing we have to "commentary" is how Blake decided to interpret the relevant passages to depict the four-part cycle. Because in the Bible, the text describes “an enormous red dragon with seven heads and ten horns and seven crowns on his heads” who descends upon “a woman clothed with the sun, with the moon under her feet and a crown of twelve stars on her head.”
The dragon embodies Satan, seeking to exact revenge on the woman who has given birth to a follower of God who will spread the Christian faith (i.e. Jesus).
What's the consensus on Blake's fixation with apocalyptic biblical themes? As opposed to a Renaissance style?
Tough question, but a good one. For Blake, the Bible was truly the most important and always-relevant, justified source of artistic inspiration, and he saw the Classical tradition (what I assume you mean by the Renaissance style) as false and pagan, as non-christian.
Was he suffering from mental health issues? Or was it just need to depict biblical themes in a new way?
I think Blake's was a deeply personal belief system, and if you look at his art works that combine text and image (such as the annotated print showing the ancient Laokoon sculpture, for example), you can really see that he does not think all images are fair game to draw inspiration from.
Did he reject the biblical works created by da Vinci, Michelangelo etc? Blake's darkness in depicting Christian themes is what draws me to his work....similar to Bosch.
"Neither character nor expression can exist without firm and determinate outlines," Blake is quoted as sayings; and you can relate that opinion to da Vinci's soft chiaro-scuro.
Why is this piece only on view once every few years?
The paints, inks, and even the paper that physically make up this artwork are quite sensitive to light; they can become fragile, discolored, or faded if exposed for too long. In an effort to preserve this piece for generations to come, it can only be in light-filled galleries for a short time with long periods of rest in dark storage in between.