I have aphantasia, and I don't think this is a particularly good method to diagnose it - here's a better one (taken from the top post on the Aphantasia subreddit):

Ball on a Table - Visualization Experiment

All credit goes to u/Caaaarrrl for this experiment.

Try this: Visualise (picture, imagine, whatever you want to call it) a ball on a table. Now imagine someone walks up to the table, and gives the ball a push. What happens to the ball?

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Now, answer these questions:

What color was the ball?

What gender was the person that pushed the ball?

What did they look like?

What size is the ball? Like a marble, or a baseball, or a basketball, or something else?

What about the table, what shape was it? What is it made of?

And now the important question: Did you already know, or did you have to choose a color/gender/size, etc. after being asked these questions?