Blue-X are a type of X Parasite that had evolved in a sub-zero environment, namely Sector 5 (ARC). These X are roughly twice the size of a normal X and have the sufficient mass to be affected by weapons. They also have bodies that maintain extremely low temperatures, allowing them to freeze hosts from within. Because of this new method of attack, Blue-X do not need to transform into a previous host in order to defend themselves.
Blue-X were mainly found at their home in Sector 5 and some parts of Sector 6 (NOC). When Samus Aran first encountered them in the latter area, the X were able to attack her by merely touching her, which allowed them to freeze Samus, who at the time was vulnerable to cold due to the Metroid Vaccine. Once Samus restored the Varia Suit function, she was able to absorb Blue-X safely, prompting them to avoid her from that point on.
- Metroid Fusion (First appearance)