There is no Shadow Weave equivalent of dead magic zones, and to understand why, it's helpful to look at where DMZs came from.
The concept of dead and wild magic is relatively new to the Realms: it dates to the Time of Troubles, when events during the Godswar tore holes in the fabric of the Weave. These holes are dead magic zones; other perturbations in the order of the Weave (inevitable when fabric under tension is damaged) are wild magic zones. In game terms, this was all introduced with the release of AD&D 2nd edition – the Time of Troubles and its events were invented to explain in-fiction the various (relatively small, in hindsight) rules changes in the new edition.
The Shadow Weave never suffered from cataclysm the way the Weave did. It was introduced with D&D 3rd edition, and it was described retroactively as having existed for eons, but mostly unknown and untapped. The sudden rise in the use of shadow magic (caused by the introduction of the Shadow Weave rules) was explained in-world as a response to the damage done to the normal Weave – spellcasters who discovered it found that there was now more incentive to use it, and Shar herself revealed it more often since she saw an opportunity to draw away some of Mystra's adherents and power base. Hence, why no Shadow counterpart to dead and wild magic was ever introduced either in-rules or in-fiction.
So, no, there is no Shadow equivalent of a DMZ. You could certainly add one if you wanted, but to be faithful to the setting it should be caused by something similarly cataclysmic to what caused the terrible damage to the Weave. The damage to the Weave is frightening and upsetting to arcane spellcasters because such scars on the fundamental fabric of the universe speak to the existence of terrifyingly powerful and incomprehensible forces that could cause them at all. If you can come up with something similar for the Shadow Weave, that would be story gold – implementing Shadow DMZs without inventing something cosmically horrible to cause them would be a missed opportunity.