Solution 1:

ok , there is probably no alternative that uses the API , unless maybe i've forgetting anything.

i can however use dimming , which is cool too, as written here:

WindowManager.LayoutParams lp = dialog.getWindow().getAttributes();  
lp.dimAmount=0.0f;  
dialog.getWindow().setAttributes(lp);  
dialog.getWindow().addFlags(WindowManager.LayoutParams.FLAG_BLUR_BEHIND);  

Solution 2:

watch out that

WindowManager.LayoutParams.FLAG_BLUR_BEHIND

is deprecated.. (at least at Android 4.0 it leads to a black screen in background...)

Reference (Source): http://developer.android.com/reference/android/view/WindowManager.LayoutParams.html

int FLAG_BLUR_BEHIND This constant is deprecated. Blurring is no longer supported.

public static final int FLAG_BLUR_BEHIND

Since: API Level 1
This constant is deprecated.
Blurring is no longer supported.

Window flag: blur everything behind this window.

Constant Value: 4 (0x00000004)

what a pitty .. :( (was a nice effect ;))