code: private void renderBlurLayer(float slideOffset) {
          if (enableBlur) {
              if (slideOffset == 0 || forceRedraw) {
                  clearBlurView();
              }
  
              if (slideOffset > 0f && blurView == null) {
                  if (drawerLayout.getChildCount() == 2) {
                      blurView = new ImageView(drawerLayout.getContext());
                      blurView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
                      blurView.setScaleType(ImageView.ScaleType.FIT_CENTER);
                      drawerLayout.addView(blurView, 1);
                  }
  
                  if (BuilderUtil.isOnUiThread()) {
                      if (cacheMode.equals(CacheMode.AUTO) || forceRedraw) {
                          dali.load(drawerLayout.getChildAt(0)).blurRadius(blurRadius).downScale(downSample).noFade().error(Dali.NO_RESID).concurrent().skipCache().into(blurView);
                          forceRedraw = false;
                      } else {
                          dali.load(drawerLayout.getChildAt(0)).blurRadius(blurRadius).downScale(downSample).noFade().error(Dali.NO_RESID).concurrent().into(blurView);
                      }
                  }
              }
  
              if (slideOffset > 0f && slideOffset < 1f) {
                  int alpha = (int) Math.ceil((double) slideOffset * 255d);
                  LegacySDKUtil.setImageAlpha(blurView, alpha);
              }
          }
      }
docs: This will blur the view behind it and set it in
  a imageview over the content with a alpha value
  that corresponds to slideOffset.