Just to clarify, there is a difference between softwood and soft wood. Softwood trees are trees that produce "naked" seeds (aka non-covered) and generally are needle bearing trees. They tend to be better for sparks and long hanging tails due to the presence of sap and other organics. There are exceptions of course, and I'm sure there are varieties that work well for BP. You'd probably want to look for relatively clean, non-sappy varieties. As a whole, they're not the best though. The opposite of this are hardwoods. These are trees that produce covered seeds (think acorns, whirlybird things from maples, etc), and generally are leaf bearing.
Soft wood on the other hand is more a description of the properties of the wood itself, and has no bearing on how it grows or reproduces or produces leaves. Hardwoods are not always hard.
Many of the highly touted BP charcoal varieties are soft hardwoods. Hardwoods tend to have cleaner wood, and the softer nature produces less dense charcoal and is probably ground up more effectively or has a more advantageous charcoal pore structure.