The "black-" in blacksmith, just means black [1], most likely in reference to the iron they worked. As far as I can tell, "Blech" doesn't have a cognate in English. If it did, it would probably by "blek" or "blik".
Blech has the same etymological roots as English blink, roots whose meaning is “to shine”. Blick (both English and German) is also related, as are blank and bleak, and, indeed, also black (which comes more from a “to burn/blaze” variant of “to shine”).
Also Blikkenslager, someone who works with sheet metal (Norwegian, Bokmål).