http://revach.net/article.php?id=2595

Section: Hanhagos   Category: Minhagim
When Do We Change Our Clothing On Motza’ei Shabbos Tisha B’Av?
Our minhag is to wear Shabbos clothing on Shabbos Chazon.  But surely we cannot wear these clothing on Tisha B’Av itself.  So when do we change when Tisha B'Av fall out on Motza[ei Shabbos?  With regard to taking off our shoes, the Rema (OC 553:2) says that when Tisha B’Av is Motza’ei Shabbos we take off our shoes after Borchu at Maariv, except for the Chazan who takes it off before Borchu after saying Boruch HaMavdil Bein Kodesh L’Chol.

Based on Rav Yaakov Emden who writes in his Siddur HaYaavetz to go to Shul in worn out clothing for Maariv on Tisha B’Av, the Minhag Yisroel Torah writes that this applies to Motza’ei Shabbos as well since you cannot daven Maariv on Tisha B’Av in Shabbos clothing, nor can you change in Shul after Borchu.  Before changing out of your clothing to go to Maariv you must say Boruch HaMavdil Bein Kodesh L’Chol just like the Rema requires of the Chazan who changes shoes before Borchu.