This is the scary point of blitzcrank and the threat and the lack of vision on Blitzcrank can really make the enemy team pretty paranoid.
Roaming is by far the best option at this point in the game as it pressures ALL the lanes and jungle as well.
Hopefully, you have shut down the bot lane and got the tower first before you do this roaming...
Ward the river or the tribush, and make sure your ADC is confident and not stupid enough to lane alone.
***(Your ADC should be freezing the lane when you roam)***
It is HIGHLY recommended you roam with a jungler with some CC
Roam with a mid laner who can push lane really hard and can have global presence or just really good for roaming.
Go Gank Mid, go ward their jungle, go steal their jungle, take dragon control.
Make sure you get a Red SightStone and ward the 3 most important places around the map. Give vision to your team for you to make plays with Rocket Grabs.
Try and catch a few members off guard, most likely their mid laner or the jungler if they are near the bot lane. Always stick with your mid laner/jungler who has CC or just take someone with you who can roam really welll in general. Be a threat, show your presence in lane and also out of lane.