There are two different ways to blend normal maps: Alpha and opacity, and normal perturbation.
When blending with alpha and opacity, the foreground normal image is composed over the background image using the alpha channel and the opacity value. The final result is normalized.
To Blend with alpha and opacity you use the Blend and Mix nodes in the Compose nodes toolbar sub menu.
When blending with normal perturbation, the normals of the foreground image perturb the normals of the background image.
To Blend with normal perturbation you use the Blend Normal and Mix Normal nodes in the 3D nodes toolbar sub menu.