The legato technique is no "real" blues technique but you can hear it very often because this technique is used by all Bluesrock guitar players.
This is a lesson for advanced players, because you need several techniques you learned before the Hammer-Ons and Pull-Offs.
 If you play legato techniques loud and fast you can play them without using the left hand.
You hit the first note and the following notes are only made by your fast hammers-ons and pull-offs.
So there´s no special rule for using your pick or fingers when you play legato licks! But take care.
Don´t play these licks to hard or with to much power.
Start slow and always relax your arm and fingers! LEGATO EXERCISE Lick 1  E||--||-| B||--||-| G||*-5-5-5-5-5h-7-*||-| D||*--5h--7-5h-7-5h-7-5h-7-5h-7-*||-| A||--||-| E||--||-| Lick 2   |-||-| |-8p--5-8p-5-8p-5-8p-5-8p-5-8--||-| |*-7-7-7-7-7-*||-| |*--*||-| |-||-| |-||-| Lick 3  |-8p--7p-5-8p-7p-5-8p-7p-5-8p-7p-5-8p-7p-5-8--||-| |-||-| |*-*||-| |*-*||-| |-||-| |-||-| Lick 4  |-15p-12-15p-12-14p-12-14p-12-15p-12-15--|| |--12--12--12--12--12--|| |*-*|| |*-*|| |--|| |--|| Lick 5-8 from Fritz Mayr  -| -| -| -| -| -| Lick5  |-13-15-13--15-13--15-13--15--|| |-15p-13--15p-13--15p-13--15p-13--|| |*--*|| |*--*|| |--|| |--|| -| -| -| -| -| -| Lick6   |-|| |-18p-15-15-18p-15-15-18p-15-15-18p-15-15--|| |*-17-17-17-17-*|| |*-*|| |-|| |-|| Lick7  -||--|| -||-8p-5-5-8p-5-5-8p-5-5-8p-5-5--|| -||*--7--7--7--7-*|| -||*--*|| -||--|| -||--|| -| -| -| -| -| -| Lick 8  S S S SS S SS S SS S SS S SS S SS S S |-13p-10-13p-10-13p-10-13p-10-13p-10-13p-10-13p-10-13p |--10--10--10--10--10--10--10- |*-- |*-- |-- |-- -|| -|| -|| -|| -|| -||