Setting Permalink on Blog Page

I have made a custom theme, in which I have set a custom front page. Posts load in a blank page i created called blog(I set it from Settings>Reading… But now when anyone click on the Blog link the url generated is site.com/blog-3, I tried editing the permalink, but it won’t set blog… it works if i use ‘theblog’ etc… what is the problem? Is there any possible solution?