Thanks for id. I don't have a description of any of the Chlorion and find them confusing, but I do have a couple of descriptions of Cha. californicum:
Males 9mm to 13mm (3/8in.-1/2in.) are typically smaller than the females at 20mm to 23mm (3/4in.-7/8in.) Both the males and females share similar body structure in that their waists are short and narrow; both having slight body bristles. The antennae and legs are black for both male and female. The wings of both the males and females are opaque and tinted the same color as the body (Hogue 1974). Blue mud wasps are not known to be aggressive and will not usually sting unless provoked to do so. They are typically a solitary species using their stingers only to paralyze prey spiders and other insects they might encounter (O'Brien 1998, Baker and Bambara 1999).

Sometimes this species is found in groups, sheltering during the night or during bad weather (Bohart and Menke, 1976).
Adults of this species feed on flower nectar, and possibly pollen. Individual wasps get most of their nutrition while they are larvae, feeding on spiders provided by their mother. Adult females capture orb-weavers (family Araneidae) and comb-footed spiders (family Theridiidae), often including black widow spiders (genus Latrodectus). These wasps capture their prey by paralyzing them with a sting. Some have been observed landing on orb webs and luring the spider out of its retreat, captureing and paralyzing the spider without getting caught in its web and becoming prey itself (Bohart & Menke 1976, Hogue 1974, O'Brien 1998).
Similar Species: Often confused with the Steel-blue Cricket Hunter which has a shorter pedicel (abdomen attachment).

I actually pulled the second N.A. species of Chalybion from Bohart and Menke (1976). Too bad they don't have species keys for some of the common groups, but I guess the book would probably come in about 20 volumes or so. Eric Eaton mentioned the head shape of Chlorion is markedly different (bigger, broader). The females, since they are fossorial, will also have a tarsal comb. On average I think they are a little larger, too.
I have the O'Brien article. If you are referring to Mark O'Brien I got it from the man himself. He basically gave me proofs of all his recent literature when I was up at the UMMZ collecting data for my spider wasp survey. There is also an interesting article about web structure as defense against attackers like C. californicum and Agenioideus humilis by a guy named Blackledge. There are apparently quite a few "spider wasps" that will go into webs to "retrieve" spiders. I think Sceliphron does the same thing. One of the most awesome examples of wasps entering spider webs is a Pompilid, Allochares azureus. They enter the webs of Filistata hibernalis, sting the spider, lay an egg on it, and then wrap the spider in its own web. The wasp actually feeds and pupates in the spider's web! Awesome behavior...for all the wasps:-)

How cool. Just reading about these wasps gives me an itchy finger. I want to photograph every behavior. I missed T. ferrugineus digging.

He gave me a proof of The Sphecine Wasps of Michigan. That's an awesome paper. It's pretty much how I want my spider wasp paper to look. He's also a really nice guy...he even suggested a group of Sphecids to do a revision on and basically hinted that it would be cool if I went to UM as a grad student.

I bought my Sphecid Wasps of the World from him. Sitting on my desk now. Helped alot with the website. Has his stamp inside it.