The less contemporary/familar a series is to western audiences the worse the dubs usually are. Inu-Yasha was a mish-mash of modern and historical but it had over-the-top character designs that made it stand out.
Abe no Seimei, I think, has been referenced before in Magical Shopping Arcade Abenobashi. If you watch enough Anime you do pick up on the Japanese History/Mythology after a while.
I have only two words for you if you want a good historical anime (exlcuding these words): Rurouni Kenshin.
Ayakashi Ayashi I found to be interesting with regards to history, but it really needs extra notes for a normal viewer to make much sense of it.
I've noticed that most Onmyoji series either are required to turn dark and stay that way (like this show does), or be dark all the way through (like Doomed Megalopolis, Tokyo Babylon and it's sequel/spinoff X/1999).