WALL STREET loves to do business in the shadows. Sunshine, after all, is bad for profits.

So it is perhaps unsurprising that players in the derivatives market want to thwart one of the worthier aims of the Dodd-Frank financial regulation: to bring transparency to the huge market for instruments known as swaps. Now some in Congress, on both sides of the aisle, are trying to block that goal, too.

Dodd-Frank focused on adding transparency to derivatives in a couple of ways. The area now under fire involves its directive that the Commodity Futures Trading Commission create rules to “promote pre-trade price transparency in the swaps market.”

The idea is that customers should get a clear picture of prices. Right now, many swaps are traded one-on-one, over the telephone. The price is usually whatever the dealer says it is.

When markets are opaque, the risks grow that problematic positions, like those that felled the American International Group in 2008, might once again create financial turmoil and spread through the system. Dodd-Frank sensibly asked that market participants provide trade and position details to regulators so this arena could be monitored better.