Is there a way for me to filter out the Blacksmith recipes that I already know? When you hover your mouse over a recipe/plan in your bags, the tooltip states that it is already learned, but the tooltip does not contain that information when viewing an item in the auction house search results.
No, unfortunately it is not possible now. Hopefully it will be possible in the future.
The only way to buy only blacksmith plans that you don't know is to make a list of all plans(here's the link that might help you) and then cross out the ones that you know. And then look for the ones that are not crossed out.