Thanks to the latest update on the Comprehensive Rules (http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/feature/266b&page=3), the following question finally has an answer:

Originally posted by Salvatore La Terra:

Player A controls an Inkmoth Nexus. He animates it, casts a Phantasmal
Image and chooses to copy the Nexus. Next turn player B casts a Blood Moon
and then targets the Phantasmal-Image-copying-a-Nexus-turned-Mountain with
a random spell, for example a Siege of Towers. Will A have to sacrifice it?

A won't have to sacrifice it. Blood Moon's effect applies in layer 4 and overrides the ability gained in layer 1 with the copy effect.

Daniel Kitachewsky
L3, Paris, France
Rules NetRep