Line Types
Blek Guide
This section aims to explain some of the most common lines that you can draw in Blek and what they are best used for. Feel free to add your own suggestions in the comments.
Straight
The most basic of lines. This is used to hit targets in a row. Note that you are able to use it when there are black circles in play, you just have to be more precise. The obvious limiting factor is that the colored circles have to be in a straight line, or you have to have two wormholes at the right point to make it work. You can also draw squares, similar to if you were drawing castle battlements, or stairs, that repeat and hit your targets.
Jagged
Jagged lines are great for hitting multiple colored circles separated by a small distance, and with potential black circles in between them. They essentially involve drawing lots of triangles on screen with no base - as if you are drawing blades of grass. There are many early levels that require this line. You can use small and fast jagged lines to go through small gaps, or large and slow ones to cast a wide field - plus a mix of the two. They are very versatile, but once the more complex puzzles come in your will have to adapt your style.
Curved
Curved lines are very similar to jagged, but have rounded edges instead of sharp. They are harder to draw precisely but are more useful for using over wide spaces. Use them when your colored circles are a reasonable distance apart and where there are black circles blocking your path.
Asymmetrical
The final traditional line is the asymmetrical one. It can be a mix of all the other lines. You can also have an asymmetrical jagged or curved line, however. Essentially, it is a line that is more complicated than the others explained so far: you may begin by drawing a shallow curve, and then follow it up by drawing a deep curve, for example. This will repeat itself and allow you to hit colored circles at various heights. It is the line that you will be using as you progress into the later game.
Wormhole
A little known technique in Blek is the Wormhole. Simply draw a circle (quickly enough that you can complete the circle before the line begins to disappear), then let go of the screen, and you will spawn a Wormhole. Draw another of these and you can send lines into one and out of the other. No levels require you to use this advanced strategy, but if you get stuck it can certainly get you out of a hole (pun intended).