Hi - I saw your post in another area and thought I'd reply here also. I am recently researching this line and came across the book you mentioned. I disagree with some of the findings of Fray Chavez for Black/Chaves note below.
The indexed marriage record is for Julian Blax (Black) and Juana Maria Chaves. I think Julian could be listed as Juliano somewhere, not William which would have been changing his name to another English name, not Spanish. If his name was William it could show as Guillermo. I have not seen the original record that holds the other info you listed for his parents and witnesses. It is also possible his middle name was William.
I think it is also possible there was a William and he was Julian's brother based on the 1860 census in Fra Cristobal; William Black 36 b MO and the next person (probably his wife Mary) 29 also b MO. Living with them are Juana 25 b NM and her and Julian's children Ana 6 and Guillermo (William) 5. One record I found looked like William J Black. Maybe there were two and they both had Julian and William as first or middle names. I need to research this more and am interested in your thoughts.
I agree with your post for the other children and marriages of this family. I'd like to larn more about Peter Pailry and Selis Noran, Santiago Hartley and Nolean Blax.