All polls are sketchy a year before a presidential election. However, even comparing sketchy polls like that, it still shows Hillary beating someone who was considered a leading Republican at the time. If you think Jeb was a bad example to use, what Republican besides the Trump phenomenon would do better among blacks? This is the group which routinely votes over 85% for Democrats.
The article about 25% of blacks voting for Trump was taken in September, 14 months before the election when most people except political junkies are not following any races. Under these circumstances, fresh new faces on the political scene get attention from the public, and Trump is no exception. However, you cannot seriously compare a poll taken when Trump is being a fresh new face and saying all sorts of attention getting things to a poll taken a week before the election when the choice between two candidates has winnowed down and become clear cut. Well see how many blacks plan on voting for Trump then, if he gets the nomination.
I should also point out that the Washington Examiner, your link, is a right wing rag and Michael Barone is a right wing hack who guaranteed that Romney couldn't miss a week before the election in 2012.
The Clintons have proven vote getting ability among black voters. We have no idea if Bernie would have anything comparable. The Clintons have also had proven success surviving the hate-filled rants and usual excesses of the Right and emerged victorious. Just as an example, in her first run for the Senate, there was a period in the summer where Clinton had a slight lead on her Republican opponent but never seemed to break 50%. This was jumped on by Hillary's critics as being VERY significant, meaning she had "topped out" and there was no more room for her to move up. She ended up winning with 57% of the vote.
Indeed, Bill's second run for Arkansas governor, (a two year term), and Hillary's rather close finish to Obama in 2008 are their only electoral defeats in several decades of politics. Bernie is a good speaker who comes from a mostly white liberal state who has not established his ability to get votes from a diverse electorate yet.
I have no idea what you plan to accomplish putting up worse case scenario after worse case scenario a year before the election. The black vote for Democrats in presidential elections increases in both percentage and turnout percentage since 1992, so going back to use the turnouts of the 1970s is just silly. That's a whole generation ago or more, the kids in junior high in 1976 now have grandchildren in junior high or older. I already proved, in a previous post, that if we go back and use past turnouts and percentages for 2012 that Obama still wins the popular vote in that election. And every election the groups which vote Democratic get bigger, and the groups which vote Republican get smaller. So the Republicans can look forward to an even more difficult time in 2016 than 2012.
Like the sons of the South trying to prove the Confederacy could have won The Civil War if only they had more men, more weapons and better strategy, so too can Clinton lose this election if nominated if the blacks decide to have turnouts and Democratic voting percentages of the early seventies. But Democratic black turnouts and voting percentage have been in an upward trend since 1974, so I don't see why using those percentages now prove anything.