Some notes about FlavourArt Blackberry, which I have some experience with, and love for.
The guide recommends 1%, which I agree is a useful starting point. The FlavourArt website currently recommends 2-3%.
Source:
Online Store :: Flavors :: Sweet flavors :: Blackberry flavor
Even at 1% it can be a distinct flavour. It can be musky and even 'smoky', as well as fruity.
If I'm aiming for a distinct blackberry flavour, I use it at 1%, but I find it really useful at lower levels, where it adds some complexity to the overall mix. It seems to be a very volatile flavour that can diminish over time, which is perhaps why FlavourArt are currently recommending 2-3%.
Used at 0.3% in a fruit/menthol mix containing 27% total flavouring, someone who doesn't like Blackberry noticed it instantly and didn't like it.
Used at 0.1% in a fruit/menthol mix containing 36% total flavouring, I can still taste it (and I don't have extra sensitive taste buds).
Used at 0.05% in a fruit/menthol mix containing 30% total flavouring, even the person who doesn't like Blackberry enjoyed the mix.