Getting an agent is hard - especially for an unpublished novelist. Your query will be rejected for a million different reasons, most of which have nothing to do with you, your story or the quality of your writing. It's not a matter of getting your query sent to 12 agents, it's getting your query to the 1 agent who happen to be looking for what you have to sell. That's tough.
By my experience, you will do yourself a favor by casting your query net WIDE - and that includes agents who are not known for or who don't even say that they are interested in historical fiction. I did not ignore the agents who listed historical fiction as an area of interest, but I did not focus on them either. I queried reputable agents who repped fiction - LOTS of them.
With a good query letter, you are just as likely to pique the interest of an agent looking to add a historical author to his list, as you are to gain the attention of an agent who, being known for repping historical fiction, is literally bombarded with it.
Publishing is a subjective, subjective, weird, subjective business. One agent may only want Tudor England settings. Another may feel the market is saturated with Tudor and is looking for Feudal Japan - and oh, that agent, he just signed with a Fuedal Japan author, and now he thinks a Medeival Mystery author would be a good addition to his list because he likes to mix it up. These needs and desires are in a constant state of flux.
And then there's the competition for attention.
My agent (who maintains a list of varied genres) gets on average 300 to 600 queries per week - and she a pretty low key snail mail only agent - the biggies get way more. Out of 300 queries she might request five partials, of the five partials, maybe one full. She takes on about one new author per year.
Th' odds are agin ye.
But don't be dismayed. I was unpublished and with no writing cred whatsoever, yet it happened for me.
My advice is grow a thick skin,
then query your brains out and find that one agent who loves your stuff.
I got to feeling all emoticonny there at the end, didn't I -