Pulsars are easiest to detect if some of the time they're pointed at us (and so are bright) and some of the time they're pointed away. This can happen because the pulsar emits radiation in a direction different from the direction it rotates. So as the pulsar rotates, it beams to different directions.
Pulsars are thought to run out of energy quite quickly and then become silent. So 99% of the pulsars that have ever lived are thought to now be silent. Astronomers and astrophysicists are surprisingly intelligent and take these facts into account when estimating the amount of matter contained in pulsars. If it were possible for sufficient matter to be contained in pulsars so as to significantly effect dark matter I'm sure they would have thought of this.
It's always fun to look in the literature to see some light curves:
The optical light curve of the LMC pulsar B0540-69 in 2009
From the above you can see that (for this particular pulsar observed in a particular range of light frequencies at this particular time) there was no significant beaming effect. It could also be that this pulsar is pointed directly at us, but wobbling slightly. Other effects can smear out a pulsar's light curve. The pulsar could be seen by its effect on nearby dust or other matter.
For a paper showing a lot of light curves, some of which do form nice beams, see:
High-Resolution Timing Observations of Spin-Powered Pulsars with the AGILE Gamma-Ray Telescope
For a paper analyzing the offset angles and the angle between the pulsar rotation axis and the earth, see:
Modeling of Gamma-ray Millisecond Pulsar Light Curves Revealed by Fermi-LAT