Sitemap helps in many ways to get seo. I saw a webmaster who was generating a sitemap in which the last update tag was daily updated, that means if you see that site map today, you will get today's date so on. And he is getting excellent seo for those pages. Is this white hat or black hat ?
you can add a script which will automatically show the current date. that doesn't mean that the site was updated today. there is no reason for which it could be blackhat and you can use it if you want
Google sitemap is to fast to improve fast crawling capability. Many of seo's put Daily update. But, Google doesn't crawl their site daily. Google has its own frequency. If you have regular updates, it increases its frequency. Similarly if your site updating daily, Google bot visits your site daily. It is not even necessary of updating your Google sitemap regularly. So, just update your site or blog. Google automatically crawl your site with in few minutes. It doesn't black hat. The usage is depends on the webmaster.