The leaves on our Jackmanii clematis are turning black . Please see attached pic
Thank you for contacting Toronto Master Gardeners with your question.
Your Clematis ‘Jackmanii’ is affected by Clematis wilt which is a disease of clematis caused by the fungus Calophoma clematidina.
There is no cure for clematis wilt and the fungus once in the soil is very hard to eliminate, but the disease does not necessarily kill clematis. Regularly remove diseased stems, be vigilant about removing all plant debris from the soil, and make sure your clematis is properly planted and well cared for. Disinfect pruning tools to prevent spread of spores and avoid transferring infected plant and soil material to a new area
If you plan to replace a diseased plant find a resistant cultivar, or choose another site in your garden for a Large Flowering cultivar.
You might find the information about clematis care helpful, see link below.
For further reading on clematis wilt, I recommend this link.