Leave it to a pro
Yes you can certainly attempt to "balance" a fan blade but you could easily end up with something worse than when you started.
Fortunately there's a "
www.fancollectors.org" club, a number of members who do such work as rewind motors, balance blades and repaint antique models as a business. Depending on what you spent, the construction of the blade, (stamped aluminum, brass or other metal riveted to a cast iron hub or even bakelite or rubber), you may decide that a slight corrective bend on one wing of a dime-store fan will make you happy or accidentally causing a crease in a rare brass blade will devalue your investment.
Along with simply bending one blade forward or back to solve your problem, there may also be an out-of-clock situation whereby a blade may have been forcefully stopped against an object, thus bending it away from the adjacent blade, harder to see but also capable of causing vibration of a different type.
One way to approach a simple bend is to remove the blade and attach it to a shaft of a drill press. By lowering the blade horizontally down toward the table you can see which blade is not in line and can bend one or all to the same pitch. Another tedious method is to leave the blade on the motor and use the front and back guards to measure distance away from perfect center with a ruler. There can also be the use of an auto timing light to isolate the offending blade, marking the spot with crayon while in motion so it can be isolated. Then there's the matter of shaving a weight off a blade or adding solder to achieve no vibration. A lot of things to think about depending on your investment and available time.
First thing I'd do is see the website. Questions are accepted from non-dues-paying members and there are people willing to walk you through the process. Best of all, it can be a useful collectible!