Engine oil to get dark is normal - nothing to be worry.
engine oils are designed to keep the dust particles in oil so that it could be drain while changing oil. If you don't change ur oil for long interval then it is possible that dust particles in oil turned into sludge in oil pan because oil no longer able to keep the dust particles.
On next change - first use engine flush. And also always change the engine oil on specified intervals only. Sometime people change oil too early and next time too late.
Also do u change oil brands and viscosity frequently?