I have separated the part and use only Aspose.Cells for Java APIs to check your issue. I have extracted the Excel file from your template PPT file using my own code. Then, I tested, I loaded the Excel file by Aspose.Cells for Java APIs and then converted the chart in the second sheet to Image file, it works fine as per the original chart in the template file. I have attached the sample output image for your reference. In the output image, there is no black background at all, the image is similar to the original chart in the template file (attached), although I have taken one page for the long image to be rendered.
Sample code:
Workbook workbook = new Workbook(“chart_transparency.xls”);
Also, I am using our latest version of the product v7.0.3.6, please try this latest fix.
If you still find any issue, we need your complete runnable code (by only using Aspose.Cells APIs) to get the issue on our side, also attach the template Excel files here. We will check it soon.