How do i replace blank lines in a file with some text using sed?

[maltiger]

Man page for sed: https://www.lightnetics.com/post/3334

Sample file is:

cat linefile
This is line one

This is line two

This is line three

Use this, ^$ mean blank line here.

$ sed 's/^$/blank this!/' linefile

Result:
This is line one
blank this!
This is line two
blank this!
This is line three