What version are you using?
The black square is possibly the program trying to print your company logo, I would check the format of the file that you have configured for your company logo, does the black square appear in the print preview?
Also check the setting in reports & forms, Invoice, to see if you have a check mark in the Company Logo setting, if it is checked try unchecking and try the print again, if you are using a custom form, this setting may not solve the problem depending on the version you are using.