Try to think of a tree's bark as you do your own skin. If you were to paint your skin with a non-permeable coating, you'd be in trouble. Same thing is going to happen to the tree if you try to spray it with a non permeable coating, it's going to be in trouble.
Sure, we've all seen trees in people's front yards that have been painted white about 6' up the trunk but... they didn't paint the whole tree and they didn't repeatedly apply coatings every few weeks as the tree grew. As an aside, I'm told this was a way for people who routinely consumed too many adult beverages to find their way home to their own house. I guess it sure would make one's home easy to spot in the dark when one's headlights shined on all that white paint.
Here are a few sites to help you understand better why Chestnut Blight is so deadly-
Excerpt from above-
The blight fungus is carried from tree to tree by small animals and insects, who pick up and transport the infected spores from tree cankers on their feet, fur, and feathers. The disease is also spread from tree to tree by weather elements such as wind and rain. Spores from an infected tree are lifted by wind or carried by rain to a healthy tree, entering it through cracks or wounds in the bark. The spores multiply rapidly and give rise to sunken cankers that expand and kill everything above the canker--usually in one season. It has been observed that a chestnut tree can die in as few as four days after being infected with the blight fungus.
Tom Volk explains things so well-
excerpt from above-
Cryphonectria parasitica is a fungus that invades the tree through wounds of cracks in the tree's bark. Once the tree has been invaded, the fungus grows through the vascular cambium of the tree, eventually girdling the tree and killing it.
Nice diagram of the life cycle of the disease-
So as you can see, it doesn't really matter whether the roots are immune to the disease or not. Once a spore makes its way in, it's pretty much over for the tree.