Blown out background - Highlight clipping weird behavior

Hi,

I am taking pictures of products and I want them on a pure white blown-out background. With the brush I am painting the background with a higher exposure and whites and I am enabling the highlight clipping (J key) to make sure only the background is clipped. However, on somes images, even though the whole background turns red (which means, if my understanding is good, that it should be pure white), I can still see some blue spots on the background. If I increase the temperature to 7000K (just for testing), the spots are even more noticeable and the background is still totally red with the highlight clipping detection. Any idea what the problem is? Is my understanding of the highlight clipping not good ?

On the other hand, when I shot the picture, I turned on the highlight detection on my Canon 7D so I could directly on my camera the parts of the background that were blown out. After importing the pictures on Lightroom and enabling the highlight clipping, nothing appears in red (J key) even though on my camera LCD screen, some parts were black and blinking, indicating it was totally overexposed. Any explanation for this?

Thanks!