Mr. N,
You are correct about the "B" in the Puerto Rico census. It stands for blanco which means white. I've searched through all 3 census' for Puerto Rico, 1910/1920/1930. I've come across the following names for race:
col-de color
b-blanco/white
mu-mulatto/mulatta

Others seen:
ch-chino
jap-japon

Denise, you and I know there are many persons of African heritage on the island. My boyfriend was very surprised to know many of his ancestors were listed as mu or col in the Puerto Rico census.
Toria