It depends on the context.
Можно "Под алым небосводом" - like poetry
If it's poem about the war, then would be better to use literal translation "Под кроваво-красным небом"
Can a native Russian speaker please translate the following phrase:
Please could a native Russian speaker please translate "Under a blood red sky" into Russian please, thank you :)
Под кроваво-красным (багровым) небом