If I'm understanding you on the idea behind your post, it's not meant to be an attack on black people or black women, but more of a criticism on the unfortunate implications on the episode, its writing, and whoever wrote it.
To people here, it looked like you were treating the very first Doctor being a black woman as a punchline to a joke, as though you were saying, "The Doctor was originally a black woman? Hilarious!"
That's going to stick as a first impression for the post and the explanations for it aren't going to make that impression go away.
It also didn't help that you were mean-spirited about the writer of the episode, like here: "If this is what Chibnall is going to give us, then I'm all for him staying around a bit longer. Please, Chibbs, give us some more funny stuff to laugh at. Please, tarnish your reputation as a writer even further. I'll be waiting with some popcorn."
If you want to say something (especially negative) about any medium, try not to involve people of whichever immutable trait they would have. Even if it's not to attack them, you might end up doing so by accident.
Also you should stay away from commenting on whether someone's trying and failing to be progressive, at least until you definitely (and I mean definitely) won't end up ranting about it.
On a different topic, I saw that you are improving in how you post in general. You dialed back your use of right-leaning lingo and you haven't insulted fans anymore. That's good!
You do sometimes forget not to type in the a-word. Try doing a Ctrl-F for that word before you post.
You still got a ways to go, but you're getting there!
That's all I could think of for now, I'll try to comment about the ranting later and hopefully in a constructive way.
If there's anything here you're confused or not sure about, say so!
Edit 2020-02-13:This is probably way overdue. I apologize for that.
It occurred to me that I don't know what you do or don't understand and though I have some general advice, I am unsure of the specifics of the problems with ranting you might have. This post is just in case you don't know.
Anyway, when you want to think about how to word things, putting a rough draft in a text doc to edit later would help! Personally, I like to use Notepad for board posts, but Microsoft Word is probably better because of spellcheck and whatnot.
As for how to sound less ranty, you could try doing these things:
Do not insult the creators, writers, etc. You can criticize their writing, but name calling, saying mean things about them, making mean jokes, etc. is off-putting and can make it seem like you got a thing against them.
Like doctorlit mentioned, add some positive points for things you didn't like and add some negative points for things you did like.
If you want some examples, you could say things like, "Some things I thought could have been done better in [thing you love/like] are [insert weak points here]"
"I do like that [thing you hate/don't like] did [strong points here]"
Try to limit the number of words dedicated to a section or a point, especially about something negative. For this point I admit I don't know the exact number of words that would come off as ranty, but I do know that dedicating many sentences or a few paragraphs to how [insert thing you hate/don't like] does [whichever kind of point] wrong can definitely come off as ranty. All I could say is try to reduce those number of sentences to "a few" and the paragraphs to "not as many".
If you already understood how to not rant so much, that's good! If not, you could try these tips out.
Once again, if you're confused, let me know.