To use the Blemish Tool, click the area you want to repair and drag the cursor to where you want to get a sample/clean texture from. You should be able to see a preview in the area you clicked first.
To use the Healing Brush, you must ⌥ (option/alt) click to define the source from where you want to take the sample, then click and drag over the area you want to repair.
For the Inpainting tool check this video tutorial: Inpainting with Affinity Photo (Beta)
