Ouput Frequency of FFT is read with respect to the bins or index output of FFT. Suppose the sampling frequency is 256KHz.
Since your FFT is 256 point, each index will correspond to 256/256 ie 1 KHz.Zeroth index represents 1 Khz, 1st index corresponds to 2 Khz and so on.
One more thing, the index output from block depends on FFT mode i.e Decimation in time and Decimation in Frequency.
If DIT is chosen, index won't be in same order, If DIF is chosen, index will increase from 0 to 255. Consult a DSP book for index arrangement for DIT and DIF.