I heard "Down in the River to Pray" for the first time in the movie "Oh brother were art thou" and I really liked it, but ever since I've find it really odd
The song is indeed very mystical, in terms of music and lyrics, but it doesn't sound very christian
the singer invites "brothers, sisters, mothers, fathers and sinners" to go "down in the river to pray", they talk about studying "the good old way" and it constantly ask two questions: "who will wear the starry crown" and "who will wear robe and crown"
Many of those symbols can be seen from a christian perspective. The river represents baptism, perhaps full body baptism and "the good old way" could be the ten commandments, but what are the robe and crowns?
The song seems to be describing a ritual were two people, a man and a woman, will be chosen to perform some kind of ceremony, while wearing ritual clothes, and standing on a river. That ritual is not christian in the slightest
This makes me suspect that maybe the song has its origins in african religions, since this song seems to have its origin in the african american communities. Is it possible that african religions had a ritual that resembled something like this? Perhaps the Serer religion of Senegal has or had a ritual like this?
This post might be better on /r/askachristian or /r/askanthropology.
Anyway, similar to other African-American Christian spirituals (like "This Train" or "I Have Decided to Follow Jesus"), the reference to "brothers, sisters, mothers, fathers and sinners" is just a general act of evangelism, or altar call. Calling each and every one to hear and obey the call to be baptized and give their lives to Christ.
The references to white robes and starry crowns can also be found in "Down by the Riverside" (along with a reference to "golden shoes", which is probably a poetic reference to Heaven's streets of gold), and they are references to the book of Revelation. At one point in the book, a woman (representing the church) is said to be wearing a crown of twelve stars. That one is debatable. More apparent is the white robes. In the book of Revelation, it is said that white robes are going to be given as gifts to the faithful, especially those who were martyred for their faith. These references are especially pertinent, as many Spirituals focused on reward for suffering, which is understandable considering they came out of an oppressed diaspora.
As far as "the good old way," this is an old reference to Christianity too. Before it was called "Christianity", the movement was just called "The Way", and in general it just denoted people who were following Jesus together, as Jesus said "I am the way".
My only field of interest is Christian history so this answer is only intended to shine light on why it might actually and fully be a Christian song. I don't have any insight into African cultures or religions to answer your question of whether it might NOT be a Christian song
To directly answer your question - it's a "Christian" song because it comes from a Christian spiritual movement. Whether there are syncretic elements from African faiths doesn't change the community that created the song in the eyes of a historian - it only impacts the theology of the song in the eyes of a theologian. There are many syncretic elements of modern South American Catholicism, for example, and although an evangelical protestant in the United States might assert they are "not Christian" theologically, to a secular historian those communities are accurately labelled Christian.
To answer the question behind your question you hint at with "The song is indeed very mystical, in terms of music and lyrics, but it doesn't sound very christian", let's break it down, line by line.
As I went down in the river to pray
Studying about that good ol' way
Although prayer here literally means prayer, as the song later explicity goes into, going down in the river to pray could also be an allegory for baptism, which were historically performed in rivers. Baptism is a persons initiation into the Christian community, and precedes a period of study and then confirmation into the church. The "Good ol' way" is just modern phraseology. The way is a common term used to represent obediance to the Christian teachings the lead to salvation. It finds its origin in a passage from the Sermon on the Mount: "Enter through the narrow gate; for the way is wide and easy that leads to destruction, and there are many who take it. For the gate is narrow and the way is hard that leads to life, and there are few who find it."
EDIT: As steveprimo highlights below, not all versions contain this explicit baptismal imagery. In light of that, it seems feasible to me that this baptismal imagery was either initially entirely coincidental, or intentionally added later as a gloss.
And who shall wear the starry crown?
Good Lord show me the way!
The one who wears the starry crown is the 'Woman of the Apocalypse' from the story of 'the Woman and the Dragon' in the book of Revelation. "A great portent appeared in heaven: a woman clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars."
Who that woman is, is a tough question. Is she Mary? Is she the Church? Is she mankind? Is she the Jewish people? A prayer for God (the Good Lord) to bless one with wisdom and show them the way is a particularly Abrahamic response to these impossible to answer mysteries.
As a completely unrelated aside, the urgent desire to understand the 'sealed' mysteries in Revelation is part of the reason why apocalyptic Christian cults are so persuasive. Knowing the answers to Revelation essentially ensures you have a fool-proof guide to salvation, and that eagerness leads many to willingly accept what seem like outrageous claims to those on the outside. That's got nothing to do with your question, but thought I'd throw it out there given the song highlights that desire to unpack one of the mysteries in Revelation.
And who shall wear the robe and crown
Good Lord, show me the way!
The ones who wear the robe and crown are the 'Twenty-Four Elders', also from Revelation. It's a similar sort of problem. Are they literal angelic beings? Are they representatives/priests of 24 rightous Christian denominations/World faiths? The author of Revelation never considers it important to explain who they are so it remains a mystery, just as the one who wears the starry crown is.
O [sisters/brothers/fathers/mothers/sinners] let's go down
Let's go down, come on down
O [sisters/brothers/fathers/mothers/sinners] let's go down
Down in the river to pray
This is an explicitly Christian call to evangelism. Calling family and sinners to come down to the river to pray to God for wisdom, coupled with that earlier baptismal allegory, is openly Christian.