How do resistances work in ES Blades?

With resistances as a percentage, incoming damage is reduced by that percentage - i.e if you were to take 100 fire damage with 15% resistance, you would take 85 fire damage instead.

Resistances as a number are simple flat mitigations - 81 resistance to frost damage means all incoming frost damage is reduced by 81.

Flat resistances are applied before percentile, so a fire resistance of 24 and 15% against 100 damage leads to 76 then 64.6 total damage (which I believe is rounded up to 65).

All bonuses stack, even if the GUI claims otherwise: the GUI only displays total flat changes when it shows a number next to damage or resistance; percentage changes are not shown on the GUI. However, they are actually added in practice. It's not the best GUI design, but that's how it works.