The Add to Cart button used by the blocks is pulled from the core WooCommerce plugin. So, changing the text in the standard WooCommerce buttons would also change it on the product blocks.
To change that label, I’d recommend using a translation plugin like Loco Translate.
These type of plugins are generally used to change text from one language to another, but they can also be used to edit text within the same language. I know it may sound strange to “translate” your own language, but it works.
Here’s what you’ll need to do once you’ve installed and activated the plugin:
1. From the admin sidebar, click on “Loco translate” option and click on “Plugins”.
2. Select the WooCommerce plugin from the list.
3. I’m assuming you are using a Spanish translation, so select that translation on the list.
4. Find the strings that you want to change with the filter box.
5. Add your replacement texts.
6. Click on Save, and then on Sync.
7. Reload your site.
Here’s a video about that plugin that outlines how it is used: https://www.youtube.com/watch?v=ZUPhsoUm-QE