Apparently true of Sherman. Not true of Grant.
One case I'm familiar with was the Battle of the Crater, where a black regiment had been specially trained to lead the attack after the detonation of the mine. In an early case of political correctness, the black unit was replaced at the last moment by an untrained white unit, which bungled the operation and turned it from a likely great Union victory into a major Union defeat.
Three of the 17 regiments that blocked Confederate escape at Appamatox Courthouse were black.
From March 1864 on, Grant was General in Chief, so all troops, including all black troops, were under his command.
He may or may not have thought poorly of them, but he was certainly willing to let them fight for him.
Grant, as President, fought more effectively for the rights of blacks and freedmen than any president of the 19th century.
Sherman was unapologetic in his racism and dislike of black soldiers and never allowed them to be part of his army as soldiers. In 1863 he said that "I cannot bring myself to trust Negroes with arms..."
When the war ended the North planned a gaudy two day parade called the Grand Review in DC. Meade's army of the Potomac would march the first day, and Sherman's army of Georgie and Army of the Tennessee would star on the next day. Grant commander in chief of all the armies of th United States oversaw the planning.
Only one "army" was not invited. They were the men of the United States Colored Troops. The reason that the hundreds of black troops were not allowed to march was because Grant specifically ordered them to stay away. Sherman had told Grant that if black trrops marched in any proximity to his army, he would pull his soldiers out of the ranks in protest. Grant agreed. Grant reorganized the black troops into the 25th corps and shipped them off to Texas just days before the Grand Review.
Sherman did not believe that black men deserved to wear blue uniforms. When a subordinate said, "They [black soldiers] can stop a bullet as well as a white man, " Sherman replied, "A sandbag is better." Contrast this with the Confederate Army, which paid black Confederates the same wages, gave them free uniforms and rations, and allowed them to march side by side with the rest of the Confederate Army, even if this was not authorized by the CSA government, which did not approve black regiments until the end of the war.
In 1862 Dr. Lewis Steiner, chief inspector of the US Army Sanitation Commission was an eyewitness to the occupation of Frederick, Maryland by Stonewall Jackson's army. He wrote that, "Over 3,000 Negroes must be included in this number. They were clad in all kinds of uniforms, no only cast of or captured United States uniforms, but in coats with Southern buttons, State buttons, etc. These were shabby, but not shabbier or seedier that those worn by white men in the rebel ranks. Most of the Negroes had arms, rifles, muskets, sabres, bowie-knives, dirks, etc....and were manifestly an integral portion of the Southern Confederacy Army."
Captain Freemantle, a British observer attached to Lee's army at Gettysburg wrote about a case regarding a black southern soldier and Yankee prisoners during the Gettysburg campaign: "This little episode of a Southern slave leading a white Yankee soldier through a Northern village, alone and of his own accord, would not have been gratifying to an abolitionist....Nor would the sympathizers Both in England and in the North feel encouraged if they could hear the language of detestation and contempt with witch the numerous Negroes with Southern armies speak of the liberators." Many blacks, slave and free served in CSA army in many positions. Henry Brown (SC) and James Clarke (GA) were free men and fifers in the CSA army. Charles Lutz was a free man of color from Louisiana. He participated in all the major VA battles, was a POW after Fredericksburg, was exchanged and wounded at Gettysburg, was a POW again and later exchanged and finally furloughed. Levy Carnine was a slave who served with three different master during the war, and became a local here for his efforts in getting mail through Yankee lines. Jean Baptiste Pierre-Auguste (free man of color) fought at Vicksburg and was paroled.
Gus Brown (colored) from Richmond said that "The Yankees didn't beat us, we wuz starved out!....I am a Confederate veteran."
James Gill (colored) of Arkansas said that "...all dem good times ceasted atter a while when de War come and de Yankees started all dere debbilment [devilment]. Us was Confederates all de while...but de Yankees, dey didn't know dat we was Confederates....When de Yankees ud come dey would ax [ask] my mammy, 'Aunt Mary, is you seen any Se-cesh [secessionists] today'? and mammy, sheud say, 'Naw-suh' eben iffen she had seen some of us mens, but when any our sojers ud come and ay, 'Aundt Mary, is you seen any Yankees 'round here recent?' she ud allus [always] tell dem de truf."
Tome Mc Alpin (colored) from Alabama said that "...dere ain't never been nobody afightin' lak our 'Federates [Confederates] done, but dey ain't never had a chance. Dere was jes' too many of dem blue coats for us to lick....Our 'Federates was de bes' fightin' men dat ever were. Dere warn't nobody lak our 'Federates."
(Quotes from Alpin, Gill and Brown taken from Slave Narratives).