Once you have chosen all the themes and keywords that you want to block, you can then select the apps in which you want to block the content. So, when one of the keywords appears in a selected app, a coloured box warns you of the possible spoiler.
Spoilers Blocker is a very interesting application, which also offers a lot of customization options. You can choose the color of the 'spoiler' box, the size of the font, and even the style of the app itself.
Requires Android 5.1 or higher