The Details 

• your component roots:

PerlAddVar MasonCompRoot "main   => /usr/share/example.org/mason"
PerlAddVar MasonCompRoot "common => /usr/share/site-common/mason"

• the common one lives in:

/usr/share/site-common/mason/blog/comp/show_entry.mhtml

• the alternative lives in:

/usr/share/example.org/mason/blog/comp/show_entry.mhtml