I just used some Shaklee Scour Off (love that product) on a damp cloth and removed the crud from the bottom of the iron.
I haven't done anything with fusible, so it had to be the starch/sizing - knew there was another reason I was not overwhelmingly fond of using the stuff (starch/sizing).
Okay - I will concede that starch/sizing does give a nice finish to a completed block. I will also concede that using starch/sizing on fabric before cutting it stabilizes it.
I still think that if a fabric is wimpy/limp/flimsy as is - I would not use it for most of my piecing projects.
I'm just irritated at myself for getting dark stuff on a block before noticing that I should have checked the bottom of my iron. sigh.