You're being asked what color shirt a person was wearing. You think it was blue, but you're not 100% sure. You say:
If I recall correctly, I think it was blue.
This is a phrase that you say when you're trying to remember something, but you're not sure that you're remembering it correctly.
"Recall" means "remember".
"If" clauses usually have a conclusion:
If you see him, say hi for me.
But in the example above, "if I recall" doesn't have a conclusion. The phrase "I think..." isn't a result of the "if" part of the phrase.
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