Would the not yet emerged Celestials have reduced energy source during the Blip - and if so, would that not have been a good reason to have the Eternals interfere with Thanos' plans?
“Would the not yet emerged Celestials have reduced energy source during the Blip?”
Yes. Ajak says that the disappearance of half of earth’s population for 5 years delayed Tiamut’s emergence.
What are the Eternals for then?
The Eternals are primarily deployed to prevent the Deviants from wiping out intelligent life on earth before it has a chance to get going.
Aside from that, they're meant to mostly leave humanity to learn how to deal with stuff on its own, which includes defending against cataclysmic threats like Thanos — and apparently includes creating cataclysmic threats like Ultron, who seems like a greater risk to continued life on earth (and thus sustenance for Little Baby Tiamut) than Thanos' original 50% off strategy, but didn’t inspire any Eternal intervention (as far as we know).
It’s implied in the scene with Ajak and Phastos that his limited technological assistance contributed to the creation of nuclear weapons, which were equally capable of wiping out most of humanity, but also weren’t something the Eternals felt they should deal with.
Presumably, in the long run, letting the Avengers (and the rest of the world) get on with things by themselves has a better chance of ensuring that humanity eventually reaches the required size. The Celestials have been doing this for quite a long time, they've probably learned a thing or two (and/or decided that proper farming is too boring to do on a cosmic scale).
Why didn’t the Eternals fight Thanos?
There’s no indication that the Eternals were aware of Thanos, or his plans. Hulk alerts the Avengers at the start of Infinity War, but they only seem to have a day or so (where they’re mainly trying to arrange an emergency Mind Stone-ectomy for poor Vision) before Thanos arrives on earth and gets the job done.
As far as we know, they didn’t hold a press conference to let the people of earth know exactly what Thanos had in mind, so there’s no indication that the Eternals would have been aware of his plan to instantly remove half of Little Baby Tiamut’s food source. (And at that point, it was only Ajak and Ikaris who even knew of Tiamut’s existence.)
Even if they had been aware of it, they might not have felt the need to do anything about it. They’ve been on earth for thousands of years, and the Celestials have been doing this for millions of years. While Thanos clearly believes that removing half the universe’s population will suddenly make the other half realise the error of their ways and stop expanding their populations, my money’s on humanity getting their numbers back up to Celestial-birthing level within a century or two in a universe where the second snap doesn’t happen — history tells us that, in general, humans quite like reproducing.
Thanks SmilyLily for pointing me to the answer elsewhere - I should have looked there before asking
In 2018, Thanos used the Infinity Stones to wipe out half of all life in the universe. This deeply affected the unborn Tiamut, and delayed the Emergence due to a lack of lifeforms to grow off of. However, when the victims of the Snap were resurrected in 2023 by the Avengers, the population was now enough to trigger Tiamut's Emergence.
I believe the movie did not make it clear whether celestials emerge one after the other or if there are even others that are in that process at the same time.
The way I understood it was that there are not several celestial seeds planted at the same time. So the snap would only have affected Tiamut. But yes, it must have affected him. The way the MCU wiki says it, the snap didn't exactly hurt Tiamut, but it did delay his emergence.