Do Blanding's Turtles die of old age?
It looks like as in fishes, the lack of an intrinsic cause of death in turtles in general is seen:


  Like in fishes, the reptiles appear to show three types of senescence. The African skink, Mabuya buettneri, shows rapid senescence similar to death at mating observed in Salmon and marsupial mouse. Most of the lizards and snakes undergo gradual senescence comparable to the pattern exhibited by a majority of vertebrates. On the other hand, turtles, tortoises and crocodiles continue to grow throughout life and are thus credited with slow or negligible senescence.
  Patnaik 1994 Gerontology 40(2–4):200–20.


The main article by Congdon, the researcher who the Discover article profiles, also seems to be fairly definitive about this lack of senescence (or negative senescence?) in Blanding's Turtles specifically:


  Survivorship, reproductive frequency and size-adjusted mean clutch size were all higher in the Oldest age group compared to the younger age groups.
  Congdon et al 2001 Experimental Gerontology 36(4–6): 813–827