Ordinarily, I cannot read historical fiction. The anachronisms just pile up and eventually, I can't enjoy any of the story because I'm fuming at the inaccuracies. So it turns out that the cure for this is to make sure your historical fiction is written by a history professor. :)
That said, there are definitely some things in Shadow of Night that are not accurate to the period.
1. The one that bothers me the most is Diana dancing at her wedding: the concept of leading and following is simply not one that is found in 16th century couples dances. The volta, the galliard, the pavane, the bransle were all dance types that came as named dances with set sequences of steps, where both partners needed to learn said sequences in order to do the dance.
2. Opals are from Australia, so the chances of an ancient Roman vampire having opals are ... well, nonexistent, really. I choose to interpret this as Diana's mistake, though: she sees mother-of-pearl and thinks "opal".
3. Pigments weren't "enriched with" gemstones. That's not how pigments work. A pigment is a material that gives paint its color; it's not something you enrich. Also, really the only gemstones that make usable pigments are lapis lazuli and malachite, and those are actually semi-precious stones, not gemstones. But again, this is easily explained as Diana's mistake, and is nicely counterbalanced with her observation that not holding the quill in a death-grip really, really helps.
4. Black Nag is a set dance for three couples, so Matthew and Diana couldn't have just joined the end of the line - they needed two other couples to do the dance with. There were other English country dances of the period that were done "long ways for as many as will", but Black Nag isn't one of them.
5. 1591 is about a generation too early for the name Rebecca to be plausible for ordinary (read: non-Puritan) English people.