Your code, setup and example worked fine on my xCode. I tried first only setting the background color to something obvious like red.
self.view.backgroundColor = [UIColor redColor];
This showed through the keyboard. Afterwards I tried an image without a blur effect.
UIImageView* img = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"images.jpeg"]];
img.frame = self.view.frame;
[self.view addSubview:img];
This to showed through, but not as much as the red because the image I used was one similar to a galaxy as you stated, and therefore had a lot of black in it.
Lastly I tried your exact code, using the gaussian blur effect to create a color.
UIColor *tintColor = [UIColor colorWithWhite:0.21 alpha:0.4];
UIColor *background = [[UIColor alloc] initWithPatternImage:[[UIImage imageNamed:@"images.jpeg"] applyBlurWithRadius:19 tintColor:tintColor saturationDeltaFactor:1.8 maskImage:nil]];
self.view.backgroundColor = background;
This to also worked, but it was extreamly hard to notice. The combined affect of dark colors being blurred once by the gaussian and being blurred again by the keyboard made it almost unnoticeable. I suggest you try a similar approach to see if you are actually getting a transparent keyboard, because if the code and info you have given are correct, then it seems like you are and maybe can not see it.
Also the keyboard is only slightly transparent, so if you are expecting a full transparency then that is not the case. This slight transparency gets minimized the more uniform the background color is for instance if you had an all black or white background it would be unnoticeable.
Edit: I confirmed that this is not the case on the iPad running iOS7.0.x I get the standard all grey keyboard, even using standard apps like Safari. It seems that Apple had not optimized the transparency for such large screens and it was causing lag issues. It also seems that this is partially fixed in iOS7.1 and there are more transparent iPad features in iOS7.1, but people are recommending turning them off as they cause the iPad to run slower than normal.