Create a WordPress block editor style variation for the cover block to turn it into a knockout effect. Screenshot: https://cloudup.com/cGrfbjVS8q5

block-editor-styles.scss

// we load the other CSS styles into the block editor and then override some of them here
// at present, the color overlay is not knocked out in the block editor
.is-style-knockout-text {
	.wp-block-cover__inner-container {
		z-index: 1;
	}
	.wp-block > * {
		font-size: 160px !important;
		font-weight: 900 !important;
		color: transparent !important;
		margin-bottom: 0;
	}
	.block-list-appender {
		display: none;
	}
}

functions.php

<?php
add_action( 'after_setup_theme', 'block_editor_support' );
/**
 * Registers the cover block style variation
 */
function block_editor_support() {
  /* Cover */
  register_block_style(
    'core/cover',
    array(
      'name' 	=> 'knockout-text',
      'label' => 'Knockout Text'
    )
  );
}

styles.scss

// see: https://css-tricks.com/css-techniques-and-effects-for-knockout-text/
.is-style-knockout-text {
	min-height: 0 !important;

	.wp-block-cover__inner-container {
		padding: 0;
		z-index: auto; // fixes a chrome bug I do not understand
	}

	.wp-block-cover__inner-container > * {
		max-width: 100% !important;
		font-family: 'Rubik', sans; // free Google font. looks great at 900!
		font-weight: 900 !important;
		line-height: 1;
		text-transform: uppercase;
		font-size: 6rem !important;
		font-size: 22vw !important;
		color: #fff !important;
		text-shadow: 2px 2px rgba(#000,.3);

    // try to prevent words wrapping
		@include mq768 {
			font-size: 15vw !important;
		}

		@include mq1100 {
			font-size: 10rem !important;
		}
	}
  
  // especially because we set color to transparent, it's really important to test for feature support
	@supports( background-clip: text ) and ( mix-blend-mode: color ) {
		background-clip: text;
		-webkit-background-clip: text;

		.wp-block-cover__inner-container > * {
			color: transparent !important;
			text-shadow: none !important;
			margin: 0 !important
		}

		&::before {
			mix-blend-mode: color;
		}
	}

}