I'm also planning to apply to Musicology PhDs for Fall 2019, so I'll share with you what I've learned so far. Just some background: I'm a rising senior that's doing a BA in Music at a liberal arts college, but I also have significant experience with performance and composition, and I hope to do a comp minor if I manage to get into one of the school that offers it.
Your background as an opera singer can be a big plus for your application if you frame it right in your apps. I am also a performer and composer, and my musicology advisor has told me to emphasize my distinctions in these areas; it really helps you stand out from the big pile of musicology applicants, many of whom have relatively little experience in the 'creative' side of music. It would be particularly beneficial if your proposed research interests align with the music you studied in your masters, or perhaps even just related to opera / vocal music in general. Have you become proficient in reading one of Italian/French/German at this point thru your singing? That would also help. I've been scouring a lot of university websites and their lists of current graduate students, and it is definitely not uncommon that there are instrument majors (though mostly of bachelors) going on to PhDs in musicology. I think as long as you have a decent background in history and theory, you are as competitive as anyone else.
If you want to become a music history prof, I've been told by many profs if you don't get into one of the 'top" programs, don't go. To beat the dead horse even more, yes "top" an erroneous concept, but indeed there are only a handful of schools that monopolize the job market for musicology phds. As far as I know, these schools are Yale, Princeton, UCB, Harvard, Stanford, UPenn, Cornell, NYU, UChicago, Columbia, UCB, UCLA, Indiana; to a slightly lesser extent, Duke, CUNY, Eastman, UNC, UIUC, OSU, Brandeis.
I couldn't tell you which ones are more likely to accept those with a performance degree. Duke offers a performance practice MA degree that you can opt to complete as part of your musicology phd, if that is something that interests you, certainly your background as a performer will be beneficial, and worth mentioning in the SOP. UChicago and Cornell also seem particularly appreciative of blurring the boundaries between performance, composition, theory, & history. I would also guess that musicology programs housed in a School of Music (ex. Eastman) rather than in the School of Arts and Sciences (ex. Yale) would consider more significantly your performance background, but take that with a grain of salt, I'm really not sure.
Good luck!