For what you’re doing, you could just use a redirect instead of incurring the overhead of mod_rewrite:
RedirectMatch /mt(.*) /blog$1
That will work on pages, images, anything with a path beginning with “/mt”.
If you want to use mod_rewrite anyway, remove the leading slash from your rule (yes, you use it for the redirect above, but not for mod_rewrite) and rearrange the parentheses:
RewriteRule ^mt(.*) /blog$1 [R]
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