Here’s a recipe to make LEDs produce slow, continuous light effects rather than abrupt changes normally obtained from square-wave drive signals.
If you are after really accurate control of one or more LEDs, the best option by far is pulsewidth modulation (PWM) which is usually obtained from a dedicated PWM chip or a suitably programmed microcontroller. On the other hand, if the blink frequency not terriblyimportant, other, much simpler methods are available. For example, get out two square-wave oscillators running at slightly different frequencies and mix their outputs together in an XOR exclusive-OR) logic gate. That’s all it takes to build a beat-frequency oscillator whose low-frequency output signal may be pulse-width modulated in
triangular(-ish) fashion.