This issue has now been resolved.
Blackboard have identified a problem with the rendering of some formula questions in existing Blackboard tests. A partial fix was rolled out on Friday 15th July which means that all newly created questions (from the 16th July) are displaying without a problem. Blackboard are now working on a fix for existing questions.
As for now, the only workaround available for this issue would be using Firefox browser, as it renders MathML natively, so the images will look well there. The WIRISplugin converts that MathML to image so browsers like Chrome that don’t support MathML can render it. You may use this option as a workaround while we wait for the fix.