I think that March Of The Black Queen forms sort of a couple with White Queen. WQ is on the white side of Queen II, MOTBQ is on the black side. In White Queen, written by Brian, I see the sad end of a wonderful relationship. In March Of The Black Queen, written by Freddie, I see the opposite: the relationship is far from wonderful, yet it continues.
I think that MOTBQ is a very sarcastic song and tells us about the power that a certain woman has over her man, and all the stupid things that this man does to please her. Sometimes he is rewarded, most of the time he is punished. Makes him question why he follows her and where she goes, from the beginning of the song.
These ups and downs are being reflected by many metaphors for alternating positive and negative feelings. Some examples about the nigger sugar and the blue powder monkeys that were mentioned earlier:
"A little nigger sugar then a rub-a-dub-a baby oil"...
Nigger sugar is an ancient expression for raw cane sugar. During the slavery period, sugar was brought from Africa, where it had been extracted from sugar-cane. This extraction process was done by the slaves, who stamped it with their feet. After this, the raw sugar was further processed to clear it. The cheapest sugar was not post-processed and thus kept its brown colour. However, people believed that the brown colour came from the feet of the slaves, and thus this sugar soon was referred to as "nigger sugar". Only the rich people could afford white sugar, and nigger sugar had to suffice for the poor. In this light, "a little nigger sugar" can be translated to a poor situation, or, can be seen as a metaphor for the dark side of the relationship. This however is followed by the "rub-a-dub-a baby oil". If we consider the "dub-a" only to be a doggerel to "rub-a", then it would read "rub-a baby oil", or "rub of (as in "rub with") baby oil". This might relate to a massage and there you have the ups and downs in the relationship.
"Blue powder monkeys praying in the dead of night"...
"Powder monkey" is a slang expression for a person who carries or sets explosives (the word comes from old navigation, where it was a boy employed on war vessels to carry gunpowder). A "blue powder monkey" would then be a sad person (as in "having the blues") whose mind is in an explosive state. He's praying in the dead of night for all of this to end.
Hey, just some thoughts :-)
Anyway, I once ran a geocities website which tried to interpret several old Queen lyrics line by line. MOTBQ was one of those and the text above comes from one of my backup files. I'll try and see if it's still online somewhere.