what does " crack thumb" and also " Balck fall" mean in this sentences from a Play: " that is the House of shaws! blood built it; blood stopped the building of it; blood shall bring it down. see here! I spite upon the ground, and crack my thumb at it! Black be its fall! if you see the laird, tell him what ye hear..."
Neither is a common phrase, but we can make a good guess from context.
By implication, cracking his thumb (I'm guessing in the same sense as "cracking your fingers") is an insulting gesture in the speaker's culture.
"Black be its fall" is an idiomatic version of "may its fall be black", and again by implication is intended as a wish that the House of Shaws shall fall in an unpleasant manner.
You didn't tell us where this came from, but I'm guessing (from the use of "laird") that this is intended to be a Scots dialect or something related to it. If you want to research the exact meaning of these phrases, that may help you do so.
(@FumbleFingers' guess that these may have been made up is a good one. It was common for authors to invent "bad language" -- curse words and the like -- to avoid printing something that might offend the reader. We still see this in television scripts, such as the invention of "frack" in Battlestar Galactica. It's also common when you want something that seems appropriate for the speaker's culture but aren't willing or able to do the research to find the correct phrase -- lazy, but common.)