So, what exactly are you trying to do? If a field is not set by default then the "<xen:else />" will run. It should work, but without seeing the actual code we have no idea.
EDIT: You can try this as well.
Rich (BB code):
<xen:if is="{$visitor.customFields.bloc_note.on} == 'on'">
hide or show this content if custom user field checked
<xen:else />
Show stuff here for people with option not checked
</xen:if>