I am trying to create a channel down the ridge of this steering spindle:
Like the channel running down this connecting rod, used to save weight:
The surface is created using a swept blend. What I've been trying is creating a new datum plane offset and parallel to the spindle. Sketching the ridge, then projecting it onto the surface. I cannot seem to indent/ extrude this sketch into the surface of the spindle, creating a channel or groove. Any help is much appreciated!
You could use the Offset feature - with draft - or expand, and use the sketch that you have created to create the offset in the sketched region (I had an internal sketch but it can be a sketch feature - drop down shown.
Without seeing a sketch of what you're actually trying to achieve, it's impossible to help.
Charlotte has the right method for a common depth throughout, although there might be a few more with a project and sweep if varying thickness is required. Even an extrude will suffice if a common thickness in the web is desired. Many ways to look at this so the desired result is important to know up front.