The plan to use the Blood Moon to delay the Reapers was successful to some extent.
The reason it was only somewhat successful is pretty straightforward.
It was because... there were just too many Reaper Fleet ships.
Duanmu Huai had initially hoped this might cause the Reapers to halt their advance. In reality, very little reaction was provoked. Just a few more warships added to deal with the Blood Moon — after all, with the vast number of warships, they could just go straight up and attack.
That was seriously annoying.
Moreover, within this interaction, Duanmu Huai observed the Blood Moon's weaknesses.
In simple terms, the Blood Moon, as an ultimate composite entity, indeed possesses considerable combat power. However, that battle power had finite limits too.
After all, the ultimate form of the Blood Moon is only about the size of a small asteroid. Of course, given its defensive power, regular warships couldn't destroy them.
The reason is quite simple: not enough firepower.