Pine is the most notorious wood for blotching. Special products are sold for reducing blotching on pine, but they are only modestly effective—even when used properly.
By far, the most effective method of eliminating blotching when staining pine is to use a gel stain. This is a thickened stain, which colors effectively but doesn’t penetrate well so it doesn’t highlight the uneven density in the wood.
Apply a gel stain the same as you apply liquid stains. Wipe or brush on a wet coat and wipe off the excess before the stain dries. Some gel stains dry fairly rapidly, so you may need to work fast or get a second person to help on large surfaces. It’s faster to wipe stains than to brush them.