A large corner of the opaque world of derivatives was supposed to be a lot brighter on Monday thanks to new rules passed as a result of the financial crisis. But some traders in the $650 trillion market for privately negotiated derivatives say they still are pretty much in the dark.

Banks and other financial firms that are large dealers of certain complex derivatives called swaps must publicly report the size, price and other details of trades under new rules by the Commodity Futures Trading Commission that went into effect...