A lot of poisons do pretty specific things, like causing muscles to go to either extreme of totally relaxed or maximum spasms, or interfering with neural transmitters. These things are pretty fundamental to terrestrial life, so immunity is pretty rare around here, but alien critters may not have muscles or nervous systems so… But still, you could well render the muscles in a region unusable, possibly permanently if the blob is not good at detoxifying…
I am not entirely clear on the specifics, but I get the impression that zeds are very dead. The blob doesn’t so much revive the original systems as it just manually operates them on a local basis. So I get the impression that there is no blood-flow to speak of, that the cardiovascular system is just blob. Now, fluids will spread through fluids, so the poison should be able to spread through a blob, but without the blood-flow the process will be very slow, especially with the blob spread out so thin. It is sort of like injecting coloured dye at one end of a long, narrow tube of water… And because you are attacking the structure of the blob directly, rather than knocking off bits as you cut up the host, the attack will make a lot more sense to the blob, so it seems pretty plausible that it would intelligently rip the poisoned bits out and throw them away where a sword through the torso would be much the same as all the other obscure engineering issues that it has been trying to figure out in order to get its ride working. It can sort of physical injuries given time, but they are strange, a pollutant in a blob’s body though, that is doing things blob-style and fighting fire with fire only works if you understand fire better than the fire does…
This, of course, all treats the blob as being fluid, it may be a bit less Euclidean than that…
But yeah, poisons would be good for terrestrial threats. But, umm, you would want to be a little cautious about eating them, I mean, I presume that they would be expended in the process of killing a critter, and cooking them would probably denature most of them anyway.