As the title says, if I have, say, a Blaziken with the Hidden Ability Speed Boost, does that work in addition to, or instead of Blaze? Bulbapedia offers no help.
No, abilities do not stack. In your example, Speed Boost would work instead of Blaze, as Pokemon cannot have two abilities at once.
Hidden abilities function no differently to other abilities; their only difference is that they can't be found on Pokemon in 'normal' circumstances e.g. standard wild encounters, and are only available in certain locations and events.
It's 'instead of' and not 'in addition to'. Hidden Abilities are not too different from normal abilities in that regard.
Hope I helped!