modify the loop to add div to every other post

I’m using an ACF Pro repeater field to serve blocks of content on a page of my site (each block has a title, some text, an image or video). I called the content using a loop in the page template.

I’d like to modify that loop to wrap every other block in a div. This is to add a blue background to every other block.

I’ve done something similar with a php counter, but I have no idea how to implement that this time around. Anyone have advice? Where would I put the php counter?

Here’s the code from my page template:

<?php if( have_rows('content_block') ): ?>

	<?php while( have_rows('content_block') ): the_row(); 

		// vars
		$title = get_sub_field('title');
		$content = get_sub_field('content');
		$image = get_sub_field('image');
		$video = get_sub_field('video');

		?>
<div class="content-block-item">

				<h4> <?php echo $title; ?></h4>
			    <p><?php echo $content; ?></p>
			    <img src="<?php echo $image['url']; ?>"/>
			    <div class="video"><?php echo $video;?></div>
			    <button><a href="mailto:[email protected]">get in touch to learn more</a></button>
		</div>

	<?php endwhile; ?>

<?php endif; ?>