It occurs to me that, since we already have the \ for line break, why not use it for “block break” as well?:
code block 1
\
code block 2
Two lists:
1. item
2. item
\
1. item
2. item
A line with just a \ would be a (non-fenced) block terminator.
Of course this doesn’t solve ambiguities in old documents (which still need to be standardized), but it would make authoring new documents simpler.