CSS to accompany the recipe on filtering the featured image and include an overlay if the post is protected.

protected_post_custom_post_thumbnail_html_css.css

/* Use this with the recipe here: https://gist.github.com/kimcoleman/faf2cd0a83f3ca7a5c6c368e371ad1f3 */
.pmpro_protected_post_featured_image {
	position: relative;
	overflow: hidden;
}
.pmpro_protected_post_featured_image img {
	filter: grayscale(1) blur(5px);
}
.pmpro_protected_post_blur_mask {
	-webkit-box-align: center;
	align-items: center;
	align-content: flex-start;
	background: rgba( 0, 0, 0, 0.6  );
	box-sizing: border-box;
	display: flex;
	flex-direction: column;
	flex-wrap: nowrap;
	height: 100%;
	justify-content: center;
	padding: 10px;
	position: absolute;
	text-align: center;
	top: 0;
	width: 100%;	
}
.pmpro_protected_post_blur_mask p {
	color: #FFF;
	line-height: initial;
	margin: 5px 0;
}
.single .pmpro_protected_post_blur_mask p {
	margin: 15px 0;
}
.pmpro_protected_post_blur_mask a {
	color: #FFF;
	text-decoration: none;
}
.pmpro_protected_post_blur_mask a.pmpro_protected_post_button {
	background-color: #FFF;
	border-radius: 25px;
	color: #000;
	padding: 5px 30px;
}