I'm possibly guilty of taking your description of "blobs" too literally? The thing about an oil-spot is that it appears more as a "ring" than your 'blob'!
For a oil-spot to assume 'blob' (an ill-defined solid black artefact in my book?) characteristics, it would require a second pollutant such as dust to stick to it, in my view. I'm also assuming that the returning 'blobs' are in the exact same location each time? Another factor for us 'aviation types' is the state of the rear lens element. Clearly, if you must
change a lens in the field, you must turn the camera off. Always face the open body toward the ground. Mount the lens asap. Ensure the lens cap has a body cap attached to it before you throw it in your bag. Sounds tedious, but failure to do this will result in filling your sensor with dust the second you turn the camera on & lift it up to the first aircraft!!
Back to oil-spots. Some Canon bodies are renowned for this issue, yours being one of them! By way of comfort I can tell you that many Nikon bodies have historically suffered too. Normally high ambient temperatures are a factor in this, but a previous repair/service by somebody using the non-approved lubricant can also cause it.
As with all things in life..."If you buy expensive, you waste some money. If you buy cheap, you waste all of it!" I use quality swabs (some 'cheapies' actually deposit more lint than they remove!), an Arctic Butterfly, the 'Visible Dust' Sensor Clean fluid @ around £15 for 8 ml (not even Gold coloured for that price?) & Isopropanol for grease & oil (Mainly to keep the brush heads clean) Remember that it's the 'Bypass Filter' you are cleaning & not the sensor per se!
I'm by no means an authority on this subject, just what I've found that works. My colleagues are suitably impressed as I clean all their cameras too!
Ps:- Safety Tip... 'Sensor Clean' contains no alcohol & is inflammable. Isopropanol is HIGHLY flammable!...so don't be cleaning your camera at a Barbecue!!
PPS:- Please struggle through the linked item below. It tends to reinforce my approach? HTH.http://www.dianedmiller.com/00tutorials ... Sensor.pdf