This page is a general discussion of alien life seen in the series. For particular alien individuals and species see individual entries.
Aliens of many different kinds were a fact of life in Blake's universe, for all that he and his followers mainly dealt with other humans.
Certainly there seemed to be few alien power-blocs in the galaxy comparable to the Federation, only scattered worlds. Within Earth's galaxy, the System was a technologically-superior civilisation of apparently-alien origin, but other than Blake's encounter with Spaceworld humanity only knew of it through a single captured ship, the Liberator.
The Federation appeared to be aware of the hostile extra-galactic civilisation in the galaxy M31 and had put in place an antimatter minefield to defend its border against it. Unlike the System's inhabitants, the Andromedans were far from humanoid in their natural form.
It is sometimes hard to tell whether the inhabitants of a planet were native to it or had evolved independently - the people of Chenga were indistinguishable from those of Keezarn, despite the former being a human colony and the latter an alien world. The issue may be further confused by the use of the word 'alien' in a legal rather than a biological sense.
In the case of Auron it is genuinely unclear whether the inhabitants were originally from Earth - the Auronar had their own mythology, and Cally was explicitly referred to as an alien by Jenna, but Avon spoke of them 'not wanting to get involved with the rest of humanity.'
It seemed that few alien worlds independently developed advanced life, but many had their own primitive vegetation and lower animals. (It should be remembered that life on Terminal evolved from Terran species and as such was not strictly of alien origin.)
Occasionally very unusual entities were encountered - the Darkness was an immaterial intelligence from another dimension, the Host was an immense organism which appeared to be an actual planet, the fluidic particles which destroyed the Liberator behaved like a viral infection, and so on.