I found some blue patches in the Arabian desert in Google Earth. It looks like as if they were lakes with sand dunes rising out of the water. But there isn't water in the desert, at least not this much, or is there? So what do we see on the image? I realize that the colour we see in the image may not be accurate.
In response to Fred's comment about whether it might show water after a rain
I took some time to browse Landsat archives and see if things are changing. Here's a comparison of the same region on two different days (note the different dates, about 1.5 months apart). The comparison shows that things are changing, i.e. it doesn't permanently look like this. Most of the time it's like the first image. This is not the exact same area as above, but it's "nearby".
Are there satellite images freely available taken at wavelengths that can clearly show water?
Update 2 Here's some daily MODIS images from the same time as the two photos above, I'm not really sure what to make of them though. See the clouds passing through 2013 Aug 3-4. The precipitation layers in the same tool do confirm rain during those two days.
Update 3 Yet another image showing more clearly what looks like lakes. Click for description and full-size image showing larger area. This is the eastern part of Rub' al Khali.
All that remains of the lakes are their eroded basins, standing up in forlorn little buttes a meter or so high, in stark white or gray against the Rub' al-Khali's infinity of rusty-red sand.
It doesn't explain the bluish colour, but it's likely these dried up lake beds that I see on the images.