Avoid background on GPUImageAlphaBlendFilter

Hi all

i'm trying to combine 2 images (same image where part of it is blurred and the rest is normal) with GPUImageMaskFilter combined with GPUImageGaussianBlurFilter.

All is fine except for filter background that is alwais present.

maskFilter = [[GPUImageMaskFilter alloc] init];
[maskFilter setBackgroundColorRed:1 green:0 blue:0 alpha:1];
gaussianBlur = [[GPUImageGaussianBlurFilter alloc] init];
blendFilter = [[GPUImageAlphaBlendFilter alloc] init];
blendFilter.mix = 1;

If I set alpha to 1 I see 100% red and the rest of blurred image.
If I set alpha to 0 I see a red border between two images.

Is there a way to set the background to nil?

Thx.

Romeo