I would say to my friends: “小黑真可爱，更是小白更可爱。” Of course this is in the case when they can see my cats. Otherwise, they would not know what kind of pet my 小黑/小白 refers to.

Your sentence is understandable but can be made more concise and natural. Here's a more natural version: 我的黑猫很可爱，但白猫更可爱。 This translates to: "My black cat is cute, but the white one is even cuter." By omitting the repeated "我的" (my), the sentence flows better. Additionally, using "更" (gèng) before "可爱" effectively conveys the meaning of "even more cute" or "cuter".