No, you do not need to know fireball - the language of "you can cast" enables you to cast it through this feature regardless of whether or not you know the spell. This invocation uses the same language as the ones that allow you to cast spells the warlock usually cannot learn, such as Dreadful Word or Ascendant Step.
To supplement this answer, note that it specifically has a spell known prerequisite of only eldritch blast, not eldritch blast and fireball. It would likely state the latter if it required that you knew fireball in order to make use of it.
Since you are planning ahead, and be warned players and long term players "Gang aft a-gley", then you need to give them lots of opportunities to pick up an item, or a cap, or be permanently enchanted by a water nymph.
These are best done as side quests, in which the apparent side quest doesn't reveal your plans.
So, the river has stopped flowing in a village. The villagers are afraid, since the water nymph Gladys has always kept the water clean and free of monsters. Jaco the pig farmer swears he's seen a troll, but he's a notorious liar. Could they help, it's worth one of Jaco's pigs and as much free lodging they ever need here?
The nymph will be found near a hole in the river bed where the water is pouring down into a dark cavern below. She is dying. "Help me, kill the troll and her evil magic will end and I can seal the hole. It'll be deep and dark that water, let me cast a protective charm upon you.."
Down they go, I suggest ropes.. and it's a dark deep lake, at the bottom of which they can just make out by the light of luminescent rocks a water troll cackling and chewing on a pig carcass. "Ah ha my pretties" she gloats, " you can't get me here, your lungs will explode and your brains will be mine to suck out of your ears".
The players should then make suitable rejoinders, a fight should ensue (remember that crushing/slashing weapons might take a disadvantage to attack underwater whereas piercing weapons are fine. No need to swim, they can breathe!
Gladys says thanks, and rewards them by making the ability to breathe underwater permanent. They get a pig from Jaco, and they can sleep at the village whenever they want.
Fun, short evening game, and your dastardly needs are met.
Best Answer
The only way for the warlock alone is through True Polymorph (possibly)
The only way for a warlock to gain access to the blink spell is through a 9th level Mystic Arcanum of true polymorph. You can use that spell to turn into a monster that has access to the blink spell innately. As of writing this, there is no official monster who can do this.
You can try a Voyager Staff
There is one magic item, found in Guildmaster's Guide to Ravnica that grants access to the blink spell. It is called the Voyager Staff, and as long as you never expend the last charge, you don't run the risk of losing access to blink.
A Ring of Spell Storing provides a refillable option1
Another magic item that could work is the Ring of Spell Storing which lets another spellcaster provide you with access:
Granted, you could only store one casting of blink at a time, but if you have a friend with the spell, this would work long term.
1: Credit to medix2