I Think that they don't cause you can probably use a spawned to get them but I haven't tried it so I'm not sure.
I personally think they don't have much value, there are still blaze spawners out there and they can be massively killed for these.
They can be farmed, I've seen a lot of them. Blaze farms use pistons or dispensers with water to let water onto the blazes to kill them.
They are valuable as a infinite substitute for fuel, and for creating brewing stands and enderchests. But mainly for fuel.