To elaborate further as to why Black Knight as an Ike echo makes absolutely zero sense and shouldn't be so much as entertained...
The basis of the suggestion comes from the fact that both Ike and the Black Knight were taught by Greil. That literally does not matter. Not only are their two sword styles canonically very different, but if having the same teacher mattered, Lilina (a mage, by the way) should be a Roy echo. After all, they were both taught by Cecilia. We could go even further! By this logic, Edelgard, Dimitri, and Claude would all work as Byleth echoes because Byleth taught each of them! Never mind how that makes absolutely no sense and is completely out of character!
But beyond that, the single biggest issue that hasn't been stated enough is that...
Black Knight is an armored unit. Ike is an infantry unit.
Fire Emblem as a franchise has four major movement types for its various classes: Infantry (foot soldiers with only light armor), Cavalry (units who fight on horseback), Flying (units who fight while airborne), and Armored (units who fight on foot with heavy plated armor).
The distinction between these four is made clear throughout the series. Infantry units are the standard but also have a wider variety of classes available. Cavalry have a larger movement range but are impeded by terrain features like forests. Flying units are unimpeded by terrain but have more common weapons that are super effective against them (bows and wind magic). Armored units are slow, have low movement, and are heavy (in games with build and constitution anyway), but have skyhigh defense and sometimes a skill like Pavise. The only time there's been any real case of overlap is with one specific class line: the Great Knight, which is both Armored and Cavalry.
Besides the aforementioned case of the Great Knight (and if you want to really split hairs, the Conqueror, which is just Walhart's personal Great Knight equivalent), these four are always portrayed very differently. This is why it legitimately makes no real sense for Ike, an Infantry unit, to have Black Knight, an Armored unit, as an echo.
Black Knight is one of the faster armor units in the series and he's still a massive, hulking monster of a man wearing enough armor to defend a small village. You would need to change so much about him to have him fit Ike, including but not limited to reducing his size, completely foregoing his own animations, being totally okay with him clipping into himself while while trying to do Ike's animations, making him significantly faster than he's ever canonically been depicted, and in general removing every last thing that made him such a memorable character.
Do you see the way his Assist Trophy was depicted? That's the Black Knight. A sable clad monster of a man who strikes fear into the hearts of those he draws his sword against, as he slowly lumbers over to them and borderline bludgeons them with a single swing of his blade. A man who doesn't so much as flinch at the swings thrown at him due to the massive weight and stability of his enchanted armor. A demon who bursts out of a village door and oops, now Oscar is dead because he and his horse got turned into mincemeat. A force of sheer terror who lets you know that you may outrun him, but not forever. And the second you let up, it's over for you. Because he will never stop his pursuit of you if you're his target.
And absolutely none of that would be retained by forcing him into an echo of Ike.
"but Dark Samus!"
Gonna stop you right there. Making Black Knight an Ike echo makes the Dark Samus situation feel as seamless as Richter echoing Simon. This isn't like Dark Samus being a Samus echo. This is like King K. Rool being a Yoshi echo.