Hi. I run a history blog as well, but I notice that whenever I tag a post "history" it doesn't show up in the tag list. Are there only certain blogs that get to be part of the history tag?

The history tag is one of those broad general interest tags (like illustration, art, music, education, gif, etc) that has this special function where certain posts get featured by tag editors, so that when you search #history, you only see the posts that were featured rather than every post on tumblr tagged “history”. You’ll know that your post was featured if there’s a little blue tag in front of all your other tags:

To see all the other posts, there’s this other option to “show all posts tagged history”.