This seems fragile as it relies on uniqueId never having been called before in the current process. Instead it might be better to test that it's numeric (or even just assigned), which will probably require breaking apart the assertions to:

expect( parsed[ 0 ].uid ).to.be.a( 'number' );
// ... etc

Introducing lodash inclines to make suggestions 😁 _.merge could avoid the nested spread operators:

const onChangeBlock = ( uid ) => ( attributes ) => {
	const index = findIndex( blocks, { uid } );
	const newBlock = merge( {}, blocks[ index ], { attributes } );
	// ...

Maybe I'm misunderstanding how merge works but I think it can cause issues if an attribute is an object. It won't replace it but merge it instead. Am I right?

You're right, but isn't that what you want? For the incoming changes to be merged with existing attributes?

Not really, If I call onChange( { myAttribute: { key1: '1', key3: '2' } } ) and I have { myAttribute: { key2: 2 } } in the current state. I don't want to keep the key2. I want to replace the attribute completely.

Mmm, as documented, properties of setAttributes are applied as a partial patch, not a complete replacement. I like to think of it akin to React's setState. Otherwise it's very inconvenient to update a single attribute of a block in an input's event handler. I'd be okay with an additional replaceAttributes perhaps though if we need to support this case.

I think you're misunderstanding my point. I agree, setAttributes is a partial patch, meaning we can change a single attribute, but I was thinking inside this single attribute if this attribute is an object.

Oh, I understand now. And you're right, that would be undesirable, so merge is a bad idea. 👍

Thinking about an idea state shape, it might be nice that when we receive the initial parsed set to transform it into a shape that more easily supports lookup and reordering, like:

{
	blocksByUid: {
		'jz1ajlkq': {
			blockType: 'core/text',
			attributes: {}
		},
		'kzjqoosz': {
			blockType: 'core/image',
			attributes: {}
		}
	},
	blockOrder: [
		'jz1ajlkq',
		'kzjqoosz'
	]
}