Is there a way to only highlight blank spaces/tabs under the marked region?
Basically if I have whitespace-mode turned off and if a select a region all the spaces and tabs get highlighted.
I'm not sure if there exists any package which can allow you to selectively highlight spaces and tabs in a region.
But what you can do as an alternative is, select the region you want to use the whitespace-mode on, narrow the buffer to this region by pressing
C-x n n (you might get a warning from emacs about using a disabled command) and enable the whitespace-mode in this narrowed region. After you are done, press
C-x n w to execute
widen command which will undo the narrowing.