This image shows the blast scene of the first explosion from above. You can see the dark spot where the bomb went off.
It is between the elliptical blood splatter on the walk and the streak of white material against the building.
I've seen bomb residue similar to the white stuff before. When a Zinc and Sulfur mixture is ignited, and it takes only
a small electrical wire glowing red hot from a battery short circuit to set it off, a large amount of cloudy white Zinc Sulfide
powder is created. Most will rise into the air, but sometimes the explosion will blow some of the unburned mixture out
against objects.
In another case Tannerite that has too much Aluminum dust can cause the dust to discolor surrounding objects in the same way.
Tannerite explosions usually have an orange tint to the cloud. Also, Potassium Nitrate and Aluminum powder create what is
called "flash powder". It can be used as a primary explosive, set off by a .22, to trigger Tannerite. But it, too, can be used
as the explosive charge and if the perp over did the Aluminum the amount of cloud and white dust covering objects will
be greater.
http://i.dailymail.co.uk/i/pix/2013/04/15/article-2309563-19505F4D000005DC-61_634x388.jpg