Alright, time to give a run down on the Spider Blood Assassin.

How exactly does one go about being a "Good" Assassin?

As an assassin, shouldn't this class have Knowledge (Local) and/or Knowledge (Nobility)? And why Knowledge (Nature)?

Weapon Proficiencies
If I remember right, one of the many issues with the Monk (in both 3.5 and Pathfinder) was that it's weapon list was so restricted. Why not just grant flat Simple Weapon Prof and then list the extras?

Assassin Techniques
Camouflage - You have a weird bit of grammar in the first sentence.

Child of Araneae - This feels a little weak to me as a 9th level feature. Maybe if the spider got half the Assassin's Sneak Attack dice.

Combat Trick - There's no spacing between this and the choice above it.

Drain the Fluids - And this works on any kind of creature? Elementals, Undead and Oozes?

Weapon Training - This is honestly a waste of a technique. Sure, normally the Assassin can't take it but the feat isn't that great to begin with. Maybe if the technique let you count as some level of virtual fighter or something.

Widow's Toxin - This ability needs to be tagged. Also, poisons in Pathfinder don't have "initial" and "secondary" damage, that was a 3.5 thing. Viper for example.

Why does this key off charisma instead of constitution?

Save Bonus against Poison
The name is honestly clunky, I'd consider renaming it. And it needs to be tagged as an ability. Also, does it apply to magical and supernatural poisons?

Advanced Techniques
You forgot to bold the name of this ability.

A Touch of Venom - The wording of this ability is a little clunky. You should word it more like, "If the Spider Blood Assassin successfully delivers an unarmed strike or a melee touch attack, they may use their Widow's Toxin Assassin Technique in addition to the normal effects of the attack."

Weapon Subtly
I think you misspelled subtly. This ability is something generally gained by a class earlier on than this point and once you add this with some of your other abilities, the class is starting to become MAD.

This class has no capstone; what entices me to take all 20 levels of this class?

In fact, once I hit level 10 there's little to keep me in the class except for some numeric boosts and if there were some more Assassin Techniques, Advanced Techniques or feats that I really wanted for my build but hadn't gotten yet by level 10.