If someone performed an action that requires a blessing before it, but did not recite the blessing, the way to proceed depends on the following. If it is a Miswah (commandment) that is ongoing, such as ציצית (Sissith) and Tefillin, where one continues to wear them after putting them on, or if it is the commandment of Sukkah, where one remains in the Sukkah for a length of time, then one recites the blessing even after one has put on the Sissith and so on.
On the other hand, if one did not recite a blessing, and the action is over, such as before a Milah, where the action is completed instantaneously, then he cannot recite the blessing. In other words, once the action has been completed, there is no more window of opportunity to recite the blessing.
(See Shulhan 'Arukh with Rama, 15:1. Kaf Hahayyim, ibid., 8)
Forgot the blessing before the action, Bracha, Beracha, Berakha