Blackline
Whether a blackline differs from a redline depends on who you ask, which is worth knowing before you assume the other side means what you mean.
What it is
A blackline is a document showing the differences between two versions. In most practice that is exactly what a redline is, and the two words are used interchangeably.
The name comes from reproduction rather than from marking up. The account usually given is that a draft marked in red pen, once run through a copier, came back with the changes rendered in black, and the term attached itself to comparison documents produced mechanically rather than annotated by hand. Which of the two words a person reaches for is a matter of where and when they trained, and of firm habit — there is no reliable rule about who prefers which.
The distinction some draw
A minority of practitioners use the two words for different things, and they are consistent enough about it to be worth knowing.
- Redline for markup a person made — the changes someone deliberately proposed, whether by hand or with tracked changes on.
- Blackline for a comparison a machine generated between two finished versions, without regard to who changed what or why.
On that reading a blackline is the more neutral document: it reports every difference, including the ones nobody meant to make. That is precisely why some people ask for one before signing.
Which word to use
Use whichever your correspondent used. Neither is wrong, the difference rarely carries weight, and a request for "a blackline against the execution version" is asking for a comparison, which is unambiguous however it is labelled.
Word itself takes a side, for what it is worth: its Compare feature calls the comparison it generates a "legal blackline", which is about as close to an authority as the word has.
Where you do need to be precise, name the versions rather than the genre: "a comparison of the 14 March draft against the version circulated last night" cannot be misunderstood, and no glossary entry is needed to read it.