Ok so, Warlocks don't technically have Spell Slots. They have Pact Magic slots. If you hover over the tab for the non-cantrip spells, you'll see it's shown in purple rather than blue. As far as I'm aware, any spells you get from your Warlock levels are cast through that, so you won't be able to cast Warlock spells using Bard spell slots.
As for the Pacts, historically Blade is considered one of the weaker Pacts. It's definitely better in BG3 than standard 5E as they've added Charisma scaling to the pact directly rather than requiring an invocation, but arguably it still suffers because of Warlock's inherent survivability issues. As was mentioned, Warlocks are kind of built around the Eldritch Blast cantrip. They get Invocations every couple of levels, several of which modify Eldritch Blast (e.g. Agonising Blast adding your charisma modifier to EB's base damage—spells don't typically scale with stats so that's kind of a big deal). Eldritch Blast essentially deals the same damage as a 2-handed weapon but with the benefits of it being ranged, and the damage of the spell increases as you hit certain levels to keep it on par with the extra attacks that martial classes get. Pact Magic is basically reserved for "Oh shyt" buttons, out of combat stuff, and particularly powerful effects.
