This.... is not a good take. The underlying premise of your argument is that because blacks -will- vote Democrat, that means that the Democrats need not consider them in political calculas.
This is wrong for two reasons:
First, and most pressing, while blacks do vote democrat overwhelmingly... they still need TO vote to count. Not considering them, or worse picking a VP candidate that actively alienates, is unlikely to pull off more than a few percentage points of the black vote and give them to trump. This is true. What your... 'analysis' seems to completely gloss over is that they can and would more likely... simply not vote. This isn't a zero-sum game, the option of refusing to participate is there and would be used.
Second, above I mentioned that only a few percentage points would be peeled off for Trump. That's more significant than your analysis admits. In fact, you almost seem to reject the idea out of hand. Here's the thing,in every swing state but Arizona, African American's make up a sizeable minority. Even a few percentage points swapping to Trump makes for a massive gain for him and massive loss for the democratic ticket in any tight race.
So no, their vote is not 'generally secure'. Its merely 'reliably blue... if they vote'.