Blocks in Rebol are not a mere "stuff" delineated by square brackets. Rather, it's a heterogeneous container that can hold first-class values and represent both code and data (homoiconic property), thus serving as a base for the syntactically-rich data format.

It also has a well-defined model [1] within the language, and, as such, can be freely operated upon at the run-time.

On a grand scale of language design, blocks form a concrete-syntax tree that can be derived from (or converted to) both textual [2] and serialized binary [3] forms.

Moreso, Rebol's syntax was in fact inspired by Logo. So, the relation between Arturo and Rebol seems to be very superficial; reminds me of Spry [4] that claims to be Rebol-like but is in fact an opinionated Smalltalk.

[1]: http://www.rebol.com/docs/core23/rebolcore-6.html

[2]: https://pointillistic.com/ren/

[3]: https://github.com/red/docs/blob/master/en/redbin.adoc

[4]: http://sprylang.se/