Context :

Aims : We present a detailed analysis of the prompt and afterglow emission of GRB 050410 and GRB 050412 detected by Swift for which no optical counterpart was observed .

Methods : We analysed data from the prompt emission detected by the Swift BAT and from the early phase of the afterglow obtained by the Swift narrow field instrument XRT .

Results : The 15 - 150 keV energy distribution of the GRB 050410 prompt emission shows a peak energy at 53 _ { -21 } ^ { +40 } keV .
The XRT light curve of this GRB decays as a power law with a slope of \alpha = 1.06 \pm 0.04 .
The spectrum is well reproduced by an absorbed power law with a spectral index \Gamma _ { x } = 2.4 \pm 0.4 and a low energy absorption N _ { H } =4 ^ { +3 } _ { -2 } \times 10 ^ { 21 } cm ^ { -2 } which is higher than the Galactic value .
The 15 - 150 keV prompt emission in GRB 050412 is modelled with a hard ( \Gamma =0.7 \pm 0.2 ) power law .
The XRT light curve follows a broken power law with the first slope \alpha _ { 1 } =0.7 \pm 0.4 , the break time T _ { break } =254 _ { -41 } ^ { +79 } s and the second slope \alpha _ { 2 } =2.8 _ { -0.8 } ^ { +0.5 } .
The spectrum is fitted by a power law with spectral index \Gamma _ { x } = 1.3 \pm 0.2 which is absorbed at low energies by the Galactic column .

Conclusions : The GRB 050410 afterglow light curve reveals the expected characteristics of the third component of the canonical Swift light curve .
Conversely , a complex phenomenology was detected in the GRB 050412 because of the presence of the very early break .
The light curve in this case can be interpreted as being the last peak of the prompt emission .
The two bursts present tight upper limits for the optical emission , however , neither of them can be clearly classified as dark .
For GRB 050410 , the suppression of the optical afterglow could be attributed to a low density interstellar medium surrounding the burst .
For GRB 050412 , the evaluation of the darkness is more difficult due to the ambiguity in the extrapolation of the X-ray afterglow light curve .