When using Domain Theme I often click through to admin/structure/block forgetting that this will take me to block configuration for the global default theme, rather than the default theme of the domain I'm on. What complications would arise from hijacking this menu path and directing it to the default theme of the active domain? You could still modify the block config of other themes via the menu local tasks.

One problem I can see is with admin_menu, it doesn't list the default theme as a sub-menu item of Structure > Blocks, so by hijacking the default menu item the default theme block config will no longer be accessible (via admin menu) from domains not using the default theme.