Hi,
If you upgrade to the latest version of mojoPortal, currently 2.3.9.4 it is possible. You can uncheck "Show Categories" in the feature settings to hide it on the blog cms page. Then from theme.skin you can enable it on the detail view like this:
<blog:BlogDisplaySettings runat="server"
UseBottomNavigation="true"
UsejQueryCalendarNavigation="true"
HideCalendar="false"
HideFeedLinks="false"
HideStats="false"
BlogViewUseBottomDate="true"
BlogViewHideTopPager="false"
BlogViewHideBottomPager="false"
ArchiveViewHideFeedbackLink="false"
HideTopSideBar="false"
HideBottomSideBar="false"
OverrideListItemHeadingElement=""
PostListForceExcerptMode="false"
PostListForceTitleOnly="false"
PostListUseBottomDate="true"
PostListDisableContentRating="false"
DetailViewDisableContentRating="false"
UseBottomContentRating="true"
DisableShowCategories="false"
DisableShowArchives="false"
DisableShowStatistics="false"
RelatedPostsPosition="Side"
ShowAuthorBioInPostList="true"
ShowAuthorBioInPostDetail="true"
ShowCategoriesInPostDetail="true"
/>
Hope that helps,
Joe