From this SO post, it seems to be due to the way inline code is evaluated.
> knitr::knit_hooks$get("inline")
function (x)
{
if (is.numeric(x))
x = round_digits(x)
paste(as.character(x), collapse = ", ")
}
<environment: namespace:knitr>
If you run blank_lines() through the hook, it returns an empty vector:
> paste(as.character(blank_lines(2)), collapse = ", ")
[1] ""
Which is also why you don't see anything in the md output.