OK, so after having played around with the colours a bit more, I managed to get a similar sort of appearance with the blur!
Previously, I was setting a barTintColor on the navigation bar appearance which had the following values:
R:17
G:63
B:95
A:1
This was fine in iOS < 7.0.3, and the output color in the nav bar (with the blur effect) was actually:
R:62
G:89
B:109
Since iOS 7.0.3, the barTintColor seems to take into account the alpha value of the color we set. This meant that the nav bar was actually outputting a solid color 17,63,95, and there was no blur effect.
The key to getting the blur effect back is setting an alpha < 1 in the barTintColor.
After lots of guess work and trying different RGB values, I managed to get the exact same RGB output from the nav (and tab) bar, using the following RGBA:
R:4.5
G:61.6
B:98
A:0.65
It does not look like there’s a magic ratio to apply to the previous color to obtain the new one.
Anyway I’ve actually rejected the binary that got approved this afternoon, and have re-submitted with these new values so that user’s don’t get an ugly app 🙂
Hope this helps.