All the mentions of blood in DELTARUNE are made by people pointing out that humans have blood (a monster talking to Kris), by someone who doesn't know that monsters
don't (Lancer and his bucket) or referring to a darkner's blood in particular (Lancer's blood, in this case, in the endgame).
Determination doesn't give monsters blood: Undyne overdoses on it in both genocide and neutral, when you kill her. It just makes monsters melt.
The timeline of DELTARUNE vs UNDERTALE is such that I
know for a fact that all of the following things in UNDERTALE are explicit DELTARUNE references:
1.
All of the fun events, as reference to the Fun Gang, which had come into form in late 2014-early 2015, including:
a. The Gaster Followers
b. Clam girl/goner clam, who tells you about her neighbor's kid, Suzy
c. The wrong number song, whose singer is calling for "G..."
d. The sound test, which features Gaster's Theme, called mus_st_him in the game files, followed up in DELTARUNE by ANOTHER HIM
e. Goner Kid, who talks about how terrifying it is that the world just goes on without them, then asks you to forget about them,
f. "Mystery Man", fun value 66, almost certainly gaster, given...
g. Nightmare, an amalgam of a snowman and Ice-E who shows up on the Monster Kids Word Search (yes, I do believe this is a DELTARUNE reference, see spoilers)
2. Room #66, Entry #17, featuring Gaster (typer #66 in the code) going on about the DARK; also in the files is battle data for Gaster, all of whose battle values are variants on 666.
3. Following up on 1b., if you've talked to Clam girl and heard about Suzy, Sans's workshop has a poorly drawn picture of three smiling people with "don't forget." written on it. Don't Forget is the name of the theme of DELTARUNE.
Here is what I
suspect is a reference to DELTARUNE, and thus may be spoilers:
All of Sans's dialogue in the endgame of both genocide and true pacifist runs, as well as everything in Sans's workshop, including the machine, as well as some mechanics of the Sans fight.
The main dialogue in pacifist I suspect is related:
just give up. i did.
why even try?
you'll never see 'em again.
Sans is the only character who can dodge your attacks in UNDERTALE; in DELTARUNE, all darkners are capable of such, or at least Susie's attacks when warned. When killed, he bleeds, unlike monsters; darkners are known to bleed, as shown by the King of Spades' threat that he'll turn Lancer into a bloody smear, as well as that Lancer seems to be under the impression that he can turn Ralsei, Susie and Kris into puddles of blood (whether it'd work on Ralsei remains to be seen).
Related: who drew the smiling people? My best guess is actually lancer. Sans is clearly from the DELTARUNE world, and my greatest piece of evidence besides "don't forget", "you'll never see 'em again" and "i gave up trying to go back a long time ago" is actually the most important character in UNDERTALE, ICE-E, who only shows up on a word search that Sans happened to have lying around. What sort of place has word searches? Restaurants. What place doesn't have any restaurants with Ice-E as a mascot? The underground. What place does? Hometown. Sans brought a word search from his original home, and I suspect that he shows it to you to gauge whether you've experienced DELTARUNE before. Unfortunately, you haven't. Cause here's another whopper: there is a character who experienced UNDERTALE before DELTARUNE, a character for whom DELTARUNE is a sequel. That character is you, or rather the SOUL controlling Kris. The dialogue options are under the impression that Undyne knows Alphys, even though someone who has only known them in DELTARUNE's world would have no reason to believe this; similarly, it's suggested that the player knows Sans, even though he's new in town. Poor Sans is backwards here, having experienced DELTARUNE first.