Can you pls provide a link to the page where you are trying to make this change? We can provide the right CSS by knowing the specific widget you are trying to customize.
All post block featured images hover have a black shadow by default so I don’t know what should I show you. e.g. mouse hover on – https://d.pr/i/MqkBya, without hover – https://d.pr/i/7VuqtH