Jelly, 18 bytes

I ended up stealing Erik's ṾṖ$İƑ¡ for this one (otherwise I'd also have 19)...

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ

A full program which prints the result.

Try it online! Or see the test-suite.

###How?

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ - Main Link: list of characters S
Ṗ                  - discard right-most (of S)
 Ṗ                 - discard right-most
   0               - literal zero
  v                - evaluate as Jelly code with right argument (0)
                   - ... b is covert-to-base, so "nb0" gives [n]
    Ḣ              - head ([n]->n or n->n)
          ¡        - repeat...
         Ƒ         - ...# of times: is invariant under:
        İ          -   reciprocation (n->1/n)
       $           - ...action: last two links as a monad:
     Ṿ             -   un-evaluate (-1->"-1" or 1->"1")
      Ṗ            -   discard right-most ("-1"->"-" or "1"->"")
             ¡     - repeat...
            Ạ      - ...# of times: all?
           ,  ”b   - ...action: pair with a 'b' character
                o  - logical OR with:
                 Ḣ -   head (S)  (i.e. if we end with 0 use the 1st character of the input)
                   - implicit print

Jelly, 18 bytes

I ended up stealing Erik's ṾṖ$İƑ¡ for this one (otherwise I'd also have 19)...

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ

A full program which prints the result.

Try it online! Or see the test-suite.

How?

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ - Main Link: list of characters S
Ṗ                  - discard right-most (of S)
 Ṗ                 - discard right-most
   0               - literal zero
  v                - evaluate as Jelly code with right argument (0)
                   - ... b is covert-to-base, so "nb0" gives [n]
    Ḣ              - head ([n]->n or n->n)
          ¡        - repeat...
         Ƒ         - ...# of times: is invariant under:
        İ          -   reciprocation (n->1/n)
       $           - ...action: last two links as a monad:
     Ṿ             -   un-evaluate (-1->"-1" or 1->"1")
      Ṗ            -   discard right-most ("-1"->"-" or "1"->"")
             ¡     - repeat...
            Ạ      - ...# of times: all?
           ,  ”b   - ...action: pair with a 'b' character
                o  - logical OR with:
                 Ḣ -   head (S)  (i.e. if we end with 0 use the 1st character of the input)
                   - implicit print

Jelly, 18 bytes

I ended up stealing Erik's ṾṖ$İƑ¡ for this one (otherwise I'd also have 19)...

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ

A full program which prints the result.

Try it online! Or see the test-suite.

Jelly, 18 bytes

I ended up stealing Erik's ṾṖ$İƑ¡ for this one (otherwise I'd also have 19)...

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ

A full program which prints the result.

Try it online! Or see the test-suite.

###How?

ṖṖv0ḢṾṖ$İƑ¡,Ạ¡”boḢ - Main Link: list of characters S
Ṗ                  - discard right-most (of S)
 Ṗ                 - discard right-most
   0               - literal zero
  v                - evaluate as Jelly code with right argument (0)
                   - ... b is covert-to-base, so "nb0" gives [n]
    Ḣ              - head ([n]->n or n->n)
          ¡        - repeat...
         Ƒ         - ...# of times: is invariant under:
        İ          -   reciprocation (n->1/n)
       $           - ...action: last two links as a monad:
     Ṿ             -   un-evaluate (-1->"-1" or 1->"1")
      Ṗ            -   discard right-most ("-1"->"-" or "1"->"")
             ¡     - repeat...
            Ạ      - ...# of times: all?
           ,  ”b   - ...action: pair with a 'b' character
                o  - logical OR with:
                 Ḣ -   head (S)  (i.e. if we end with 0 use the 1st character of the input)
                   - implicit print