On a site named "Philosophy", I'm initially inclined to consider a history-of-philosophy tag to be redundant, but I very much take your point that there is an important distinction to be made between the history of philosophy itself and philosophy about history.
My conclusion goes off in a slightly different direction, though. I'm not yet convinced that we should have a tag for questions about the history of philosophy at all. None of the questions that currently hold either the history-of-philosophy tag and the historical tag actually benefit from that tag. In some sense, anything that isn't happening right now is part of the "history" of philosophy. Given that it could be applied to nearly every question, it becomes meaningless. There's no reason to include a "history" tag on a question about Nietzsche's thought, or Hume's rhetoric. As far as I'm concerned, it's a meta tag, which I discuss here.
I suggest eliminating history-of-philosophy completely. There are much better and more descriptive tags that can be applied to questions.
And I think that we should retain the history/historical tag, but reserve it for questions that are actually about the philosophy of history. That is, questions which are themselves about history.