I don't see why eldritch blast wouldn't be an option for the War Caster attack of opportunity.
A ranged spell attack would be made at disadvantage if you're within 5ft of your target, so it's usually not a great spell to choose for a War Caster's opportunity attack. However, the attack of opportunity in this build is made at 10ft thanks to picking polearm master and using a reach weapon.
Warcaster lets you cast a spell on an Attack of Opportunity that can only target a single creature.
After lvl 5, Eldritch Blast can target multiple creatures and is no longer usable with Warcaster.
[Edit: Sorry, I was incorrect on this. I think I was confusing it with the errata for the Twinned Spell metamagic.
https://www.sageadvice.eu/2017/10/21...youre-level-5/ ]
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