Section 4.2 ATX Headings states:
Leading and trailing blanks are ignored in parsing inline content
and then gives
# foo =>
<h1>foo</h1> as an example.
The term “blank” appears in the spec (v0.27) 85 times; 83 times in the phrase “blank line[s]”, once in the phrase “blank HTML comment”, and once in the above-quoted statement.
Earlier, in section 2.1, the spec takes care to define the terms “blank line”, “whitespace character”, “whitespace”, “Unicode whitespace character”, “Unicode whitespace”, “space”, and “non-whitespace character”. But the term “blank”, in reference to a character, is nowhere defined, which renders the above-quoted statement ambiguous.
I think that it’s clear from context and from precedent that “blank” should here be treated as synonymous with “space”, but I believe it would be prudent to make that explicit in the text.