There are many factors to consider when choosing a scroll saw blade for your project. Thickness and hardness of wood as well as intricacy of the pattern are all things to be weighed.
The first thing you must look at is the intricacy of the pattern. You will need to chose a blade that is small enough to be able to cut in the tightest areas of your pattern without giving you too much trouble. Generally with thicker and harder wood you will want bigger blades however the blade you use must be able to turn within the radius of the sharpest corners of your scroll saw pattern.
Blade Size Numbers
Scroll saw blade sizes are listed using numbers. The higher the number,
the bigger the blade is. The numbers can range from as large as #12 to
sizes below #0 such as #2/0 (pronounced two aught), #3/0, etc. Generally
you won't be needing anything that's smaller than #3/0 as they are
usually considered jeweler's blades.
Scroll Saw Blade Types
There are many different types of blades. I will touch on the more common types as well as list their strengths/features.
Regular Tooth - The basic scroll saw blades. Relatively high TPI (teeth-per-inch).
Hook Tooth - Teeth have a positive rake and blade cuts aggressively. Great for thicker material.
Skip Tooth - Like a regular tooth blade but with about half the TPI. Slower cutting, better chip removal. Blade will stay cooler.
Double Tooth - A hybrid of regular and skip tooth. Teeth are in groups of two followed by a gap.
Reverse Tooth - Like a regular tooth blade except the last inch of teeth are pointing in the opposite direction. This makes for cleaner cuts on the back of your work to minimize burrs and reduce sanding time afterward. These teeth can sometimes catch on the wood on the upstroke if you aren't holding the project down tightly or if you aren't running the saw fast enough. When mounting these blades, position them so that only about two reverse teeth cut into your wood when the blade is all the way up. Reverse blades often have a skip tooth configuration.
Full Reverse Tooth - Every third tooth in the opposite direction. Has the virtually the same behavior as the regular reverse tooth.
Crown Tooth - Similar to the full reverse tooth blades, crown tooth blades have every second tooth in the opposite direction.
Spiral - The most "different" of all blades. These blades are effectively a normal blade that has been twisted into a spiral so that all sides have teeth. They cut equally well in any direction and thus require a little bit of practice to get used to them. Having teeth on all sides means they will make much wider cuts (thicker kerf lines) however you can cut bigger pieces on the saw more easily because you don't have to constantly spin your work piece around on the table. Highly detailed portrait patterns are best cut with spirals
Flat End Spiral - Same as spiral but with flat ends for easier installation in the saw.
Reverse Spiral - Same as spiral but with every third tooth or the last inch of teeth in the opposite direction for cleaner cuts and less burrs as illustrated in both examples above.
Metal Cutting Blades - Higher TPI for cutting metal
All in all, as you work with the different types of blades and experience them first-hand, you will be able to better choose the blade that is right for your scroll saw project and also one that you feel most comfortable working with. I hope these general guidelines will be helpful in getting you started and choosing that perfect blade.