Is the blastopore imaginary?
From Merriam-Webster:

Definition of pore
1 :  a minute opening especially in an animal or plant; especially :  one by which matter passes through a membrane

The blastopore is a real thing, it is not imaginary. It is the point where the mouth or anus eventually develops. I'm guessing the reason your zoology class referred to it as imaginary, however, is probably because it isn't really a pore (and especially it is not a pore into the blastocoel). It sort of looks like a pore, but it's just a little indentation (and may even be considered a blastopore before the indent forms); it doesn't really become an opening until later, and the process of gastrulation doesn't actually involve anything going inside the blastopore, it's just a folding in of the cells on themselves.
In summary, I think your understanding is sound, good job, and I just wouldn't get too hung up on the &quot;imaginary&quot; part.