RFC30: Refactoring blob (#2850)

## Motivation and Context
This is a sub-PR of https://github.com/awslabs/smithy-rs/pull/2615

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Refactors `blob.rs` file.

## Description
Some test was failing due to unnecessary import; This PR fixes it.

## Testing
NA

## Checklist
NA

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Co-authored-by: Zelda Hessler <zhessler@amazon.com>