OP I've had a batch that sounds pretty much just like you described- it was dense and had a dark purple color to it but lacked smell other than a faint grassy smell. When smoked it wouldn't stay lit in a joint, it would burn for a bit but would keep going out even when I smoked it in a joint with a few people. Also it would leave a dark black as that had a slight shimmer to it, like glossy almost. It got us stoned, but I remember stopping 1/4 of the way through with some buddies that we've smoked endless sessions together. I remember feeling fucked, but not in a good way- I felt high but it was becoming more and more unpleasant and uncomfortable.
After a few sessions I decided to try making edibles out of it since it was pretty much practically unsmokeable. I can't remember the exact process I used but I remember boiling it in water and buttter for a few hours and when I strained the ground up weed out of the water I was left with dark green grounds and a purple water. Ya purple water. I remember tripping out a little off of it and then pretty much decided to throw out the whole batch. The edible were weak as fuck if I remember right, but then again I am pretty sure I never ate much of them at all and tossed the majority of them.
It sucked, but I learned an expensive lesson that week. The smell is a very strong signifier of how potent the bud is going to be. I'm sure there are ways to fake it as well, but there really isn't a cost effective way to fake both look and smell of good weed. At the time I was pretty sure I had been conned and even called up the guy I had got it from with a full on accusation, even told him he better bring the cash back haha. Looking back though I'm pretty sure it wasn't flushed right, probably grown with some shit like miracle grow, and blasted with some purple food coloring or some shit to give it the dark purple color at the end of the flowering cycle/ during the curing process. I doubt it was cured and kept very well either after all that.
There are reports(mostly in the UK from what I've seen) of people spraying weed with things like tiny glass beads, lead, sand, and probably a range of other cheap household products to give the buds more weight, but I have never heard of or seen first hand someone who has gotten weed laced with another drug. I can't really picture a drug dealer putting other drugs on shitty weed, especially in todays world with weed being one of the cheapest street drugs and most available. I can see some sleezy mother trucker dusting a pound of shit weed with lead dust so he can make an extra 300 bucks or whatever. Or maybe more likely some would-be grower who started growing to help out his financial struggles only to muck it up and put himself deeper in the hole, who then loads his buds up with purple food coloring right before dropping them to give his mostly failed crop some sort of market appeal(yeah I thought about that one a lot when I was angry lol).
