To create a blurry background in a painting, you can follow these steps:
Choose the right paints: Use free-flowing acrylic paint in a few different colors. Dilute the paint to a mix of one part paint, one part painting medium, and one part water.
Create the background shape: Apply a basic shape of color for the background and wait for it to dry.
Blur the background: Use a clean, soft, and dry brush to gently brush over the background. This will create a blurry effect. You can either do this after blocking in each object or wait until the entire background is blocked in and then brush over it. Be cautious not to blend the paint too much, as acrylic paints dry quickly.
Continue with the rest of the painting: Once the background is blurred, continue with the rest of your painting, focusing on the foreground elements.
Remember that practice makes perfect, and it might take some time to achieve the desired blurry effect in your paintings. Experiment with different techniques and methods to find the one that works best for you.