The Crimson King is a maple that holds dark purple leaves throughout the year (not counting winter), I was wondering if this would make a good Bonsai?
No. The leaves are much too large, and don't reduce in size well in bonsai conditions. Only small-leaved maples are much used for bonsai; if you want something with black leaves, try some of the Acer palmatum cultivars.
Crimson Queen isnot the best for Bonsai. Try Mikawa Yatsubusa, Kamagata, Sharp's Pygmy, Shishigashira and more. Wildwood Maples, Sooner Plant Farm, and Pacific Coast Maples at http://www.pacificcoastmaples.com have Japanese maple bonsai.
Would a variety possibly work? I was thinking of the one called 'Crimson Sentry'. Seems the leaves on ones I've seen are comparable to the bigger Japanese maple leaves. But personally, I'd prefer a Japanese maple or vine maple any day. I think vine maples are under-rated many times.
Crimson Sentry isn't as deeply colored and also often mildews heavily on streets and other dry planting sites here. Perhaps one in a bonsai tray might turn out to be quick to mildew if watering ever got at all behind.
