Lyric discussion by caryatis 

Cover art for River lyrics by Joni Mitchell

I think the 'best baby' referred to here is definitely a lover, rather than a child, if you read the context. Little Green is the song that deals explicitly with the daughter she put up for adoption.

In Hotel California by Barney Hoskyns it points out that Blue was written very quickly after Joni's break-up with Graham Nash (of Crosby Still Nash and Young fame, and the Hollies). Apparently it was partly her jealousy and dedication to her music that drove them apart, though she loved him deeply.