It's not possible for to say if it's a comma splice, because the sentence is too poorly formed. As it's written, the comma is the least confusing of its elements. Without rephrasing, it doesn't make sense no matter how you look at the comma.
First of all, I've had it explained to me that spiritual is referring to a countable noun (a spiritual song), and not being used as an adjective for one of the poems.
Components of the sentence are in the wrong places. Among other things, this leads to confusion over what is being honoured. If something is the root, then something singular is being referenced as the object of honour. If it's something singular, you would think that it's referring to the book itself.
However, that doesn't make sense. Bryan is the author of the book. It would be highly unlikely for him to honour his own book. It makes more sense that his book is honouring the twenty-five poems and one spiritual. So, when he writes the root he must really be referring to the singular collective of the twenty-five poems and one spiritual.
This leads me to interpret the sentence in this way:
In Bryan's introduction to the book Ashely Bryan's ABS of African American Poetry, he honours its twenty-five poems and one spiritual by writing that they are "the root of Black song and poetry."
Note that the rephrasing has resulted in the removal of one of the commas as well as changing the position of the other.
Even understanding this as the meaning, commenting on the possible comma splice in the original is not useful. If forced, I would say there is no comma splice. But there's definitely a lot else wrong.