You can use a direct dye over the top to make a darker blonde. If you can get it, Wella colour fresh is available in a load of natural shades.
I like to mix my own, and Adore is my dye of choice (it lasts longer and is much cheaper than the wella) They only make browns, not blondes, so you will have to dilute with conditioner. ATM I am using Mocha (which is a warm brown) mixed with Medium Brown (which is ash/green based) and i can make many different blonde shades with just these two dyes. It depends on how warm or ashy you want your blonde to be really.
Just mix in a bowl, add as much white conditioner as you will need to cover your hair, you will only need a few teaspoons of the dye for a blonde shade. What you see in the bowl will be the final colour, direct dyes don't develop over time. Try on a strand test. By leaving on the hair for several hours you will get a long lasting colour with no damage to your hair.