QOwnNotes (https://github.com/pbek/QOwnNotes) may be an option as it is pure Markdown and need not have both the code and preview windows open. I used it before moving to Obsidian. It has some options to customise its window views, but it is a long time since I used it, so not sure if it has that block mode you want. Logseq I seem to recall worked with blocks as it is an Outliner that does save in Markdown format, but I did not like that the outline blocks added characters to the file format that slightly broke compatibility with standard Markdown formats. But blocks was Logseq’s strength.
I was thinking more of its native file format it saves to. It said “import from Markdown” which seemed to suggest it is not saving all in Markdown (otherwise would have said opens and saves to). But maybe it is just badly worded.