You could say eternals is a flop domestically but not internationally.
If it make 400million worldwide or more overall it's not a flop,but it is flop domestically because the budget is 200 million and it will make less then 200 million.
Black widow you could say is a flop domestically if you only counting box office and worldwide if you only going by box office but if add the disney plus numbers then it's not a flop.
The only other mcu movie you could say was a flop worldwide and domestically so far is the The Incredible Hulk (2008).
The Incredible Hulk (2008)
Budget
$150,000,000
Other costs etc..
$150,000,000
All together
$300,000,000
It needed to make $300,000,000 to not be a flop and it made less then that.
DOMESTIC (50.9%)
$134,806,913
INTERNATIONAL (49.1%)
WORLDWIDE
$264,770,996
This has be one of the main reasons that another hulk movie was not made because two were made within the same decade and both flopped plus they have deal with universal and that mess.
Some examples of other movies that are flops domestically this year -
Dune (2021)
No Time to Die
Jungle Cruise
The Suicide Squad (2021)
These certain anti-mcu and disney youtube channels act like the mcu never had a flop before and a rotten score(captain marvel audience score that was reviewed bomb).
Both hulk movies flop and one was in mcu and hulk was way more known then eternals but they never bring that up.
Plus eternals first film as made more then The Incredible Hulk (2008).
Hulk's first mcu film and last one.
Eternals will make more then Captain America: The First Avenger (2011).
A superhero way more known then then the eternals too.
If eternals do flop(making less the 400m) at the box office they will make up for it in other ways.