Using a WordPress blog with a WIX website

I have someone who uses WIX for their main website. Not good for SEO, I know, but very easy for them to use so they want to stick with it. They have asked me to do a blog and naturally I want to use WordPress.

I am a relative beginner with WP but I think I know enough to handle it. My question is, what will be the best URL for the blog? For example their current website is http://www.communitiesofprayer.com. What is the best URL I should try to get? Would http://www.communitiesofprayerblog.com be what I should try to get?

Not an elegant way to do things, but not sure how else to work it out. The nav menu will take them from one site to the other, but that should be OK.