According to the Pacific Bulb Society, Iris confusa "is similar to Iris japonica and Iris wattii. It is larger than I. japonica in all parts, but is very similar. At bloom it forms a stem beneath the fan of foliage that can reach 2 ft from the ground to the base of the foliage. Flowers are similar to Iris japonica in pale blue or near white."
In contrast to this, "Iris japonica , native to Japan, is very common in cultivation and seen as both nearly all white or pale blue color forms. The foliage is semi-glossy and forms fans without any visible stem."
I guess the only major difference is in the stem or lack of stem.
