Edit you get_node_from_parameter macro like this:
<override_macro name="get_node_from_parameter" dot_parameter="do" requires="servlet">
<n.catch_exception. id="get-node-block">
<n.get_node_from_request_parameter.do/>
</n.catch_exception.><n.handle_exception. for="get-node-block">
<n.exception. name="node_not_found">
<n.redirect_to url='Your Address' />
</n.exception.>
</n.handle_exception.>
</override_macro>
Replace "Your Address" with your main blog 404 page. Use "http://" at the beginning.