"Blast" is slang for a very exciting or pleasurable experience or event. So it means that the clerk was very happy to be interviewed. But people (especially actors and celebrities) aren't always sincere when they say this. It may be just a cool way of saying something polite to end the interview, like "thank you" or "my pleasure".

"It's been a blast "= I've had a great time.

The clerk was happy. When you've spent a great time w/ someone you say "it was a blast" or " I had blast" Robbie Williams uses the expression in one of his songs. "No regrets", I think

It's a colloquial expression meaning that the speaker really enjoyed themselves or had a lot of fun, so the clerk in that situation wasn't annoyed at all :)

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