Hi again, Suze, and thanks very much for your additional information. Thanks for adding your location.
My heartfelt condolences on the sudden loss of your cat. I know how difficult this for you, your family, and for your other cat, as well.
I can fully understand why you had a problem locating information on this type of spider; I just spent a long while researching it, until I found the proper information. The false black widow spider, also called the 'steatoda grossa' (Cobweb spiders (Theridiidae)) does NOT have the dangerous toxin which the genuine black widow possesses. However, it's bite CAN cause some local or systemic effects. While it's possible that this spider did bite your cat, the fact that he had a scale 1 heart murmur and seemed fine, it's also possible that your cat experienced a heart attack, which is not uncommon and often unexpected. Very often, a heart condition may be present, but no symptoms will be obvious. The fact that he had a heart murmur to begin with, indicates that a sudden cardiac event could have caused his demise. The only way to know if a spider bite was the cause of his death, would be for the vet to do a necropsy (autopsy for animals) and also, to find any fang marks from a spider bite/injection of venom. Also, a necropsy will establish a definite cause of death from any other condition, like a heart attack. It might also be possible that your cat ingested a toxin which caused this terrible event.
Here is more information on the false widow spider:
I'd suggest that you hire a professional exterminator who specializes in spiders and make sure your home is clear of any more of these, or other spiders, so you and your other cat are not in danger, IF this spider did bite your cat and caused his death.
Again, I'm so very sorry to hear this sad news about your cat, and perhaps this poem will help to ease your grief, somewhat: