Having the same project opened in both, Visual Studio and Blend, I use the former to write code while I edit the XAML in the latter.
When I add an UI element to the .xaml file and save it in Blend, if I have that file opened in Visual Studio too, I see the message that some external changes have been done and asking if I want them to take effect in the opened file*. I accept it, and I can see the changes take effect in the .xaml file in Visual Studio.
The problem is, that although the new elements have been added to the .xaml file, I'm not able to access them throug Intellisense in the code-behind .cs file.
For example if I add following element to the MainPage.xaml using Blend:
<Button x:Name="MyButton" Content="Button"/>
MyButton.Content = "Hello!";
I have just check your problem actually it is not a issue. everything is fine so what you have to do is save your xaml page on which you have added the button after accepting the changes that was done in blend..(do control + s on your xaml page one more time it will work fine)..sorry for my bad english..