First, Late Blight doesn't produce yellow leaves so that isn't the issue.

Second, it you go over to the Tomatoes forum (linked top of this forums page) you will find literally 50 discussions about leaves turning yellow.

Most of the time the problem is over-watering. Beyond that it all depends on how old the plant is and which type - determinate or indeterminate.

Dave

a few yellow leaves are no cause for concern
even if you overwatered a bit
What's a few leaves out of the whole plant? I can relate to your concern. We try so hard to get gardens growing and following directions sometimes it's just nice to know to remove a few blemished leaves and move on. This year our cucumbers have had a lot of yellow lower leaves at the same time they're blooming and setting fruit. Our cool overnight lows are the cause.

my climate is rainy until 4th of July, so even in raised beds plants can get overwatered by rain :o)

This year tomatoes have done just fine with those perforated red plastic mulch sheets. It might be a gimick, but the leaves on the tomatoes are nearly perfect. I couldn't say that in previous years with straw mulch.