If this was film (or had a conventional shutter), then wouldn't this just be the exposure time being longer than the time it takes the blades to traverse their own width?
According to the NASA info sheet, they're 1.2m across and do 2,400rpm - I reckon that means the tip is doing a touch over 542km/h, or a fraction under 151m/s.
Even if the exposure is 1/1000 of a second, the blade tip will have travelled 15cm: if the blade is 20cm wide, then its image would appear to cover 35cm, with only the central 5cm looking solid, and the 15cm on either side appearing translucent - but the centre of the blade, with a lower rotational velocity, would appear more solid on the film.
But it ain't film, and the whole blade appears evenly, so clearly it's a different effect ...