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So disclaimer, I've never used Codex: Space Marines and I only have Deathwatch info from C:IA.

In C:IA, it says Deathwatch Veterans can take "Power Weapons". I originally assumed this meant Swords, Axes, Mauls and Staves. Power Fists are listed separately for some reason.

Anyway, I encountered another school of thought that says this means Veterans can also take Relic Blades. I'd never seen Relic Blades before, but Lexicanum confirms that they are, indeed, power weapons in the fluff. They're not listed anywhere in the main rulebook though. So exposition aside, if a unit is listed as being able to take "Power Weapons" does this allow them to take Relic Blades?

No. Relic blades may be power weapons in the fluff, but they are not included under power weapons in the rules.

When referencing army list entries, "Power Weapon" is defined on page 179 as being either Power Swords, Lances, Mauls or Axes. Although there are other weapons that use the same technology (lightning claws and power fists being examples), those are the four that are the option when listed as "may take a power weapon for X points".

This is partly due to trying to keep older codexes valid during the transition to 6th edition (from 3rd to 5th, there weren't the different weapon profiles for axes/swords etc. but simply the generic term "power weapon").