已知数列\(\{a _{n} \}\)是各项均为正数的等比数列，\(a _{3} = \dfrac {1}{16}\)，\(a _{1} -a _{2} = \dfrac {1}{8}\)，数列\(\{b _{n} \}\)满足\(b _{1} =-3\)，且\(1+b _{n+1}\)与\(1-b _{n}\)的等差中项是\(a _{n}\)．
\((\)Ⅰ\()\)求数列\(\{b _{n} \}\)的通项公式；
\((\)Ⅱ\()\)若\(c _{n} =(-1) ^{n} b _{n}\)，\(\{c _{n} \}\)的前\(n\)项和为\(S _{n}\)，求\(S _{2n}\)．