The key words are "believe" and "some". I also believe, in fact I know, that "some" blacks served in the Confederate army. The points of contention are how many, in what roles and where are the documents? Those of us who "know" a very small number of blacks were in combat roles and we "know" that large numbers were in support non-combatant and civilian roles and we know that there is almost no official documentation, i.e. service records and pension files for blacks in Confederate service, other than in these civilian/non-combatant roles. We also know that some of the most respected Civil War historians, including Edwin Bearss, calls the claims of large numbers of Black Confederates as, excuse me, "B.S." James McPherson, Robert Krick, Gary Gallagher and a host of others share this opinion.
This claim of large numbers of Black Confederates is no more history than the Northern claim that the war was fought to free the slaves. I guess it "feels good" to promote this idea, it's just not true.
"All we want are the facts, ma'am."
Detective Joe Friday from the TV show "Dragnet"