"Cup that holds the blood.." makes no sense

There's a mention in this movie that the 'Holy Grail' is a 'cup that holds the blood of Jesus Christ'.

Then Indy says something about it being a bedtime story and so on.

However, when we actually see the cup, there's no blood in it. Why not?

How can they say it holds the blood, if there's no blood in it? Also, how can anyone's blood, even the Messiah's (after all, it's not the physical body or the blood that makes someone the Messiah, it's the soul), not dry up after thousands of years?

In any case, why even say that it holds the blood, if it would not only be impossible, but also, we aren't SHOWN it to hold any blood? Remember - SHOW, don't tell!

To add, wouldn't that be a RIDICULOUSLY easy way to 'cheat' by simply choosing the cup that has blood in it over the empty ones, and that's how you know it's the actual grail?

Of course the guardian could fool people by pouring blood in ALL of the cups, but that opens up a whole other Pandora's box of where would this knight get all that blood, and how could he keep it from drying up?

Then again, this whole 'knight guardian' thing raises so many questions anyway, so it's beyond the scope of this post, but this 'holds the blood'-lie is just so ridiculous, I don't understand why they put it in the movie, when it OBVIOUSLY not only makes no sense, but it CAN'T make sense.

Sigh.. or at least show us the cup ACTUALLY HOLDING THE BLOOD, if you are going to make that claim. I am so tired of movies trying to have things both ways - that never makes any sense.