Thank you for contacting Syncfusion Support“Query #1: how i can make the black corners transparent?”
We can make the black corners as transparent by setting AllowTransparency property of the ChromelessWindow to true. When this property is enabled, MS window background will become transparent .We have also prepared a sample which demonstrates the same. Please download the sample from the below link
Please try our solution and also let us know if you need a patch for this fix? So that we would provide assistance according to your reply.
Please let us know if you need any other assistance