This is currently not possible. I can say that this was requested by many people, and the team is aware of this problem.
I know this is annoying, but you can stop the problem by closing the XAML files in Blend before editing (them) in Visual Studio. Requests appear only for open files, so when you return to Blend after working in VS for a while, it will not ask you. Then you can open the file and see the changes. This works both ways: since I usually do my XAML work in Blend, I try to save the XAML files in Visual Studio.
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