Huh! I don't see Foster doing that. And as of April 1865 no Blacks had been enfranchised anywhere in the South, though the Louisiana Constitution empowered the Legislature to do so if it thought fit.
At a guess, he orders the military commanders in the South to enrol all adult males who can read and write a section of the US Constitution. I understand that was a popular test at the time. He might also order that Union soldiers over 21 (present or former) be excused the test. Iirc, even Andrew Johnson recommended something along these lines to the Southern Provisional Governors, though he was too "states rights" in outlook to make it an order. That creates a significant Black vote, though not big enough for control of the State governments. He may well also be slower than Johnson in handing out pardons to prominent Rebs, who will thus play less of a role in any elections.
Also, if Foster has called Congress into Session [1] the Civil Rights and Freedmens Bureau Bills are probably passed a year earlier. If so, Foster is most unlikely to veto them as Johnson did. They had the support of almost all Republicans, not just Radicals, and a veto would make him "a President without a party" much as it did AJ. So he will have power to prevent any "Black Codes" or other provocative moves. In short, Congress and the White House will be in reasonable accord, and you won't get the cat fight which opened the way for the more drastic Reconstruction measures of 1867.
[1] He might do so to allow the House to elect a Speaker, so as to provide a "backup" successor in case anything should happen to him. Alternatively, though, he might be content to do what Chester Arthur was to do in 1881, ie sign (but not issue) an order for the recall of the Senate, with the date left blank, so that if necessary there would be a legal way for it to reconvene and choose a new President pro-tem.