It's unlikely that a summer Tour du Mont Blanc will be cold enough to require a leg base layer even if it snows.
However, if you became injured or stranded somewhere overnight, you'll be grateful that you've an additional layer to put on.
The usual rules for base layer material apply, no cotton, and go for a synthetic material that will wick perspiration away from the skin.
As we already had running trousers that fitted the bill we took those. We've worn them as a base layer in snowy conditions before and found them very snug.
If like us, you use running trousers, remember to remove the instep straps as they would rub inside your boots and cause discomfort.