There is no definitive answer to this question, as it largely depends on the context in which the poster is used. Generally speaking, however, most bloggers would likely not be okay using a movie poster as part of their blog design – unless it was used in a purely decorative or historical context.
In most cases, using movie posters as part of blog design may come across as excessively commercial or gimmicky. As such, it may not be the best idea to go down this path.
Ultimately, the decision of whether or not to use a movie poster in your blog design is up to you. If you feel that it would look good and add an element of sophistication to your blog design, then by all means go for it! Just be sure that you are aware of the potential risks involved and that your readers will be okay with it.