Direct2D histogram effect doesn't output any image, it builds an array of floats that you can get back from the effect's D2D1_HISTOGRAM_PROP_HISTOGRAM_OUTPUT property

d2dContext->CreateEffect(CLSID_D2D1Histogram, &histogramEffect);
histogramEffect->SetInputEffect(0, ... SomeInput ... );
d2dContext->BeginDraw();
d2dContext->DrawImage(histogramEffect.Get());
d2dContext->EndDraw();

// The histogram data is only available once the effect has been 'drawn'.
int histogramBinCount;
histogramEffect->GetValue(D2D1_HISTOGRAM_PROP_NUM_BINS, &histogramBinCount);

auto histogramData = new float[histogramBinCount];
histogramEffect->GetValue(D2D1_HISTOGRAM_PROP_HISTOGRAM_OUTPUT, 
                           reinterpret_cast<BYTE*>(histogramData), 
                           histogramBinCount * sizeof(float));