Nevermind i have found the solution !
Here is how to make buddypress working withe Sage 9 blade template
in your sage theme folder/ressources/
Create a 'buddypress' folder
in your new ressources/buddypress/ folder, create a file 'buddypress.php' with this in it :
<?php echo App\Template('buddypress');
in in your sage theme folder/ressources/views/
Create a file called 'buddypress.blade.php' with this code in it :
Thats it, now Sage 9 is compatible with Buddypress
With this, you don't have to change the default template for buddypress pages like 'members' or groups' defined in wp admin Buddypress settings.
Hope that helps someone else!