You've included bash and sed tags in your question. Since I can't seem to come up with a way of doing this in sed, here's a bash-only solution. It's likely to perform the worst of all working solutions you might find.
The following works with your sample input:
$ while read -r x; do [[ -z "$x" ]] && cat boilerplate; printf '%s\n' "$x"; done < src
This will however insert the boilerplate before EVERY blank line, which is probably not what you're after. Instead, we should probably make this more than a one-liner:
#!/usr/bin/env bash
y=true
while read -r x; do
if [[ -z "$x" ]] && $y; then
cat boilerplate
y=false
fi
printf '%s\n' "$x"
done < src
Note that unlike the code in your question, this doesn't store your boilerplate in a variable, it just cats it "at the right time".
Note that this sends the combined output to stdout. If your goal is to modify the original file, you'll need to wrap this in something that moves around temporary files. (Note that sed's -i option also doesn't really edit files in place, it only hides the moving-around-temp-files from you.)
The following alternatives are probably a better idea.
A similar solution to the bash one might be achieved with better performance using awk:
awk 'NR==FNR{b=b $0 ORS;next} /^$/&&!y{printf "%s",b;y++} 1' boilerplate src
This awk solution obviously reads your boilerplate into a variable, though it's not a shell variable.
Notwithstanding non-standard platform-specific extensions, awk does not have any facility for editing files "in place" either. A portable solution using awk would still need to push temp files around.
And of course, the following old standard of ed is great to keep in your back pocket:
printf 'H\n/^$/\n-\n.r boilerplate\nw\nq\n' | ed src
In bash, of course, you could always use heretext, which might be clearer:
$ ed src <<< $'H\n/^$/\n-\n.r boilerplate\nw\nq\n'
The ed command is non-stream version of sed. Or rather, sed is the stream version of ed, which has been around since before the dinosaurs and is still going strong.
The commands we're using are separated by newlines and fed to ed's standard input. You can discard stdout if you feel the urge. The commands shown here are:
H - instruct ed to print more useful errors, if it gets any./^$/ - search for the first occurrence of a newline.- - GO BACK ONE LINE. Awesome, right?.r boilerplate - Read your boilerplate at the current line,w - and write the file.q - Quit.
Note that this does not keep a .bak file. You'll need to do that yourself if you really want one.
And if, as you suggested in comments, the filename you're reading is to be constructed from a variable, note that variable expansion does not happen inside format quoting ($' .. '). You can either switch quoting mechanisms mid-script:
ed "$file" <<< $'H\n/^$/\n-\n.r ./TATTOO_'"$currn"$'/top.txt\nw\nq\n'
Or you could put ed script in a variable constructed by printf
printf -v scr 'H\n/^$/\n-\n.r ./TATTOO_%s/top.txt\nw\nq\n' "$currn"
ed "$file" <<< "$scr"`
