Straight corn whiskey is corn whiskey aged for at least two years in new uncharred or used barrels.
That's not what Elmer is talking about.
He means unaged corn whiskey, part of the non-whiskey component of the blend.
The Seagram's Seven label says "contains 60% grain spirits." The word "neutral" does not appear. It's not vodka. The spirit has a little flavor.
All ten of the Four Roses bourbons are combined for Four Roses Yellow Label. Single Barrel is, obviously, just one of the recipes. Small Batch is a mix of three or four of them, as is Bulleit, although they are all from the high rye mash.
Although in practice it can be complicated to get the taste of a blend just right, in principle they are simple. Take a good-tasting bourbon (or other straight) and dilute it with alcohol that has not been aged.