I think that Mythal killing the Titan, likely with powerful magic, corrupted its blood into red lyrium, which is in fact the source of the Blight. The dwarves have long claimed that they think the origins of the darkspawn was in the Deep Roads from the beginning and if the Magisters had any connection it was they released it, not were the cause of it. When they thought they had opened a portal to the city in the Fade, whether Golden or Black, in fact they were entering the body of the dead, corrupted Titan. The Golden City in the Fade was a reflection of a pure Titan and the Black City what had now been revealed.
The is an interesting codex in the Decent called the Wellspring. It talks of dwarves tunnelling until they broke through into the sky. This could mean ordinary dwarves tunnelling down until they reached a Titan, which means that is the Wellspring, or it could mean ancient dwarves tunnelling until they broke through the Veil. When the veil rips it is in the sky and Skyhold is so called because it was the place where the sky was held back (veil created?) but it could be that there was a Veil originally and when it was breached, the spirits that became the elves were able to leave the Fade and enter the material world. When we fall into the deep in Adamant and the Inquisitor opens a portal to the Fade we end up there with some people up side down and then we see the way back as another hole in the sky of the Fade. In one part of the Chant the Maker is referred to as the Wellspring of All and lives in a Golden City shining with light. So either the Wellspring is found in the Fade, as she suggests, or she was in contact with a Titan.
When Solas threw up the Veil, he did not create it from scratch but was merely repairing it to what it had once been. The Titan may even have given him his idea how to do this. In this version the reason the Titan becomes upset at the Breach is that it is afraid that the old enemies of the Titans are returning. When the Evanuris originally conquered the Titans, not all may have been corrupted but the elves started to occupy them. Solas' account of what the ancient elven cities were like, sound very much like the interior of the Titan.