First, welcome to the forums.
Second, you need to get some books and read them. There is a ton of information available on this site and in the stickies at the top. Also
www.knifehow.com has a lot of info. That said, there is no replacement for sitting down and reading a couple of books on the subject before you start a project like a knife. Everything you read on the Internet is not necessarily good information. That is why this site is so good. The peer review of many experts gives a good chance of accurate information. Even then some bad advice slips through. Also, knife folk lore and knife urban legends will exist forever. No attempt by experts will make them go away. You will read about "Edge Packing", "Mystery quench mixes that will make 1030 super hard", and that all saw blades are L-6, car springs are 5160,etc. It would be nice if the world was so simple, but it is not.
I don't know where you got your info, but a circular saw blade is most likely NOT O-1. It may be a lot of things, most of which don't make good knives. It was a learning piece, so that is OK. The thing about not heat treating your first knife is to not do it yourself, have it done by someone who knows how. Preferably with them showing you how it is done.
As to the farriers rasp knife. Many modern hoof rasps are just case hardened steel and not good for knives. The ones that are usable will have to be annealed and shaped (grinding or forging) and the heat treated to harden and temper them. You will do yourself a BIG favor to buy a $20 piece of steel (O-1, 1080,5160) and work with a known material. Files, rasps, car springs, saw blades, etc. ,MAY make a good knife, but then again they MAY NOT !! After you have made a hundred or so knives you will know the difference, but for now you don't, so stick with known steels. Get a couple of knife supple catalogs, there is a lot of info as well as supplies in them.
The oxide on your blade may be from a lot of things you did in making it, but a knife should not change color after it is made unless it is exposed to some chemical (water, acid in food, blood,etc.) My guess is that it is just the rust and oxidation forming on a steel that is not high carbon. The blades of circular saws are usually a low carbon steel with lots of alloy elements to make them tough. Cheaper blades are often plated with a chrome coating to prevent rusting. The tips of the teeth are usually carbide, or impulse hardened.
You will learn more on finishing blades when you study up on the subject. There are many ways to color a blade and control oxide and rust. The steel selection is an important part of the process.
I suggest you get a copy of "The $50 Knife Shop",by Wayne Goddard. It is a great starter book for those on a budget.
My final comments are ones I address to most new members of this forum....
FILL OUT YOUR PROFILE
We don't know where you live or who you are without it.
You could be my neighbor or you could be in South Africa...We don't know.
You could be a chemical engineer or a 8th grade student....We don't know.
The info will help a lot in your getting help and good advise.
For instance. When several members joined this forum and were interested in getting started, They was helped by having books, steel, tools, etc. sent to them by other members who were interested in helping them learn. If you are not giving us anything to work with, you are less likely to get more than a simple answer.
There are many posters on this forum, some give advise based on thousands of knives completed. Others give advise on the knife they PLAN on making ,or the two or three they have made. You will soon be able to tell these apart, but in the beginning, listen to the general masses before jumping off with just one persons advise. (The number of posts is NOT a sure sigh of a good source)
Again - Welcome - Stacy
Rough Eh?
Blueing the knife will help as the blueing is actually a thin layer of "rust" and as that steel has been oxidized it is more difficult to rust.