I just partitioned my external SSD, installed MacOS El Capitan, 1Password and Dropbox (no third party startup items other than those), and then installed Blocks 3.1 Build4 and tested the same was as shown in my video. I got the crash right away. Below is the crash log. This test was done on my late 2015 5K iMac. We cannot say the iMac is at fault here, in light of the fact I also get the crash at home on my mid-2015 MBP.
Blocs3.1-Build4_CrashLog_ElCap.txt.zip (21.7 KB)