Loving "Isle of Blood", but I agree that both sound good. Also, this would be the best thing ever. I have read those books so many times and pulled new things from them each time, so I would be soooo excited for a third! Just lent the first one to a kid I worked with this summer (who loves Pirate's of the Caribbean but isn't generally much of a reader) because he asked me to pick a book for him, and he's enjoying it thus far as well.
In my humble opinion, Isle of Stars sounds like a continuation, where Isle of Blood sounds like a conclusion;) you seem to
Love Trilogies, but the series holds much potential for adding to. You could develop personal threads of each character in so many ways, but a gripping conclusion where it all pulls together could be great as well. Whatever you decide will be amazing, because you always let God lead! ;)
Isle of Stars sounds like it would be a location with something of value: treasure, magical beings, or possible a place of mental illumination. The Isle of Blood sounds like it would describe a more sinister place, nearer to the doors of the Lake of Fire or some other scene as bleak. Both would be great titles; I simply think that the title would vary based on the direction that you would want to take the book.
I'm so glad that you are writing more "Isle" pirate books! Being a pirate and sailing fanatic myself, this series is, unsurprisingly, my favorite of all your books that you've written. It has also been my inspiration for writing my own Christian pirate/adventure series. My series is still a work in progress, but I don't think I would have gotten this far if it weren't for your books as inspiration. I love your books and am looking forward to reading more!