A variety of Cobalt Blue which can sometimes be named Cobalt Violet. It is rarely found in artist paints. From what we have found it seems to be more frequently found as a reddish violet. We've seen it crop up in a few watercolor blends, but it is hard to find in paints. Sometimes it is part of Cobalt Blue Deep blends.
More common cobalt blues are PB28 (which can range from teal to middle blue to deep) and PB72 for Cobalt Blue Deep.
PB73 is rated as excellent for lightfastness, with all 8's on the blue wool scale. This is also rated at "B" for toxicity so it is potentially hazardous, handle with care and always seek the SDS and consult health and safety experts for details.
One supplier mentioned PB73 has a near-infrared reflectance, which makes it a cool pigment.
