General: Update code for readability and inclusion

There are two pieces in here:

1) The update to change blacklist to blocklist is moved to disallowed_list. "Block" has a meaning in our code, and there could be ambiguity between this code and code related to blocks.

2) This improves backwards compatibility for code that was accessing the now deprecated code.

Previously: [48477], [48405], [48400], [48121], [48122], [48124], [48142], [48566]

Props: desrosj, SergeyBiryukov, johnjamesjacoby
Fixes: #50413


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git-svn-id: https://core.svn.wordpress.org/trunk@48337 1a063a9b-81f0-0310-95a4-ce76da25c4cd