It works with no blueprint in the scene, it’s enough to create a bp based on the c++ class with the niagara component.
As you can see the class defaults pick up the custom niagara component’s parameters and can even be edited and remembered. (overwrote the yellow with a new color to test it, if I revert to default it returns to the particle’s yellow color)
Maybe some stale data cache is causing some problems? Try deleting the temp dirs and recompiling.