Blet (verb) in the OED as follows: intr. To become ‘sleepy,’ as an over-ripe pear, a special form of decay to which fleshy fruits are subject. Hence bletting vbl. n.
1835 LINDLEY Introd. Bot. (1848) II. 257 After the period..of ripeness, most fleshy fruits undergo a new kind of alteration; their flesh either rots or blets. Ibid. Bletting is..a special alteration. 1864 Reader 21 May 653 The decomposition..of the pericarp begins with fermentation, and, after having passed through the intermediate stage of bletting to use Dr. Lindley's word, ends in the total obliteration of the cellular structure.
Also, blet as a noun has this additional information: In Webster (where the only authority cited is Lindley's use of the verb). But this would not give ‘A decayed spot on fruits,’ as erroneously stated, but, That form of decay which is commonly called ‘sleepiness’ (in which there are no external spots to indicate the change).